Difference Between Test.StartTest and Test.StopTest

Defining classes of test methods with the isTest annotation
Use the isTest class annotation to define classes that only contain code used for testing your application. If your test methods are contained within their own classes and the Apex class only contains test methods, it is ideal to use the isTest annotation.

Test.startTest/Test.stopTest
There are two additional system static methods provided by Apex. These methods, Test.startTest and Test.stopTest, are used when testing governor limits. Or in other words, executing test scenarios with a larger data set.

The Test.startTest method marks the point in your test code when your test actually begins. Each test method is allowed to call this method only once. All of the code before this method should be used to initialize variables, populate data structures, and so on, allowing you to set up everything you need in order to run your test. After you call this method, you get a fresh set of governor limits for the remainder of the test until you call Test.stopTest.

The Test.stopTest method marks the point in your test code when your test ends. Use this method in conjunction with thestartTest method. Each test method is allowed to call this method only once. After calling this method, any post assertions are done in the original context

@isTest 

public class sampleTestMethodCls
{

      private static testMethod void testAccountTrigger()   { 

      //First, prepare 200 contacts for the test data

       Account acct = new Account(name='test account');

         insert acct; 

       Contact[] contactsToCreate = new Contact[]{}; 

       for(Integer x=0; x<200;x++)
        {

      Contact ct = new Contact(AccountId=acct.Id,lastname='test'); 

      contactsToCreate.add(ct);

          } 

     //Now insert data causing an contact trigger to fire. 

     Test.startTest(); 

    insert contactsToCreate; 

    Test.stopTest(); 

     }

 }

Test Class for Standard Controller

@isTest 
public class ExtensionTestClass 
{
    static testMethod void testMethod1() 
    {
        Account testAccount = new Account();
        testAccount.Name='Test Account' ;
        insert testAccount;
        
        Test.StartTest(); 
        ApexPages.StandardController sc = new ApexPages.StandardController(testAccount);
        myControllerExtension testAccPlan = new myControllerExtension(sc);
        
        PageReference pageRef = Page.AccountPlan; // Add your VF page Name here
        pageRef.getParameters().put('id', String.valueOf(testAccount.Id));
        Test.setCurrentPage(pageRef);
        
        //testAccPlan.save(); call all your function here
        Test.StopTest();
    }
}

Test Class for Controller class:

@isTest 
public class ControllerTestClass 
{
 static testMethod void testMethod1() 
 {
 Account testAccount = new Account();
 testAccount.Name='Test Account' ;
 insert testAccount;

 Test.StartTest(); 

  PageReference pageRef = Page.AccountPlan; // Add your VF page Name here
  pageRef.getParameters().put('id', String.valueOf(testAccount.Id));
  Test.setCurrentPage(pageRef);

  myController testAccPlan = new myController();
  
  //testAccPlan.save(); call all your function here
 Test.StopTest();
 }
}

Test Class for StandardSetController:

@isTest 
public class TestStandardSetController 
{
 static testMethod void testMethod1() 
 {
 List <Account> lstAccount = new List<Account>();
 
 Account testAccount = new Account();
 testAccount.Name='Test Account' ;
 lstAccount.add(testAccount);
 Account testAccount1 = new Account();
 testAccount1.Name='Test Account1' ;
 lstAccount.add(testAccount1);

 insert  lstAccount;
 
 Test.startTest();
  Test.setCurrentPage(Page.YOUR_PAGE);
  ApexPages.StandardSetController stdSetController = new ApexPages.StandardSetController(lstAccount);
  stdSetController.setSelected(lstAccount);
  YOUR_Extension ext = new YOUR_Extension(stdSetController);
 Test.stopTest();
 }
}

Please follow below salesforce Best Practice for Test Classes :-

1. Test class must start with @isTest annotation if class class version is more than 25
2. Test environment support @testVisible , @testSetUp as well
3. Unit test is to test particular piece of code working properly or not .
4. Unit test method takes no argument ,commit no data to database ,send no email ,flagged with testMethod keyword .
5. To deploy to production at-least 75% code coverage is required
6. System.debug statement are not counted as a part of apex code limit.
7. Test method and test classes are not counted as a part of code limit
9. We should not focus on the  percentage of code coverage ,we should make sure that every use case should covered including positive, negative,bulk and single record .

• Single Action -To verify that the the single record produces the correct an expected result
• Bulk action -Any apex record trigger ,class or extension must be invoked for 1-200 records.
•  Positive behavior : Test every expected behavior occurs through every expected permutation , i,e user filled out every correctly data and not go past the limit .
• Negative Testcase :-Not to add future date , Not to specify negative amount.
• Restricted User :-Test whether a user with restricted access used in your code .

10. Test class should be annotated with @isTest .
11 . @isTest annotation with test method  is equivalent to testMethod keyword .
12. Test method should static and no void return type .
13. Test class and method default access is private ,no matter to add access specifier .
14. classes with @isTest annotation can't be a interface or enum .
15. Test method code can't be invoked by non test request .
16. Stating with salesforce API 28.0 test method can not reside inside non test classes .
17. @Testvisible annotation to make visible private methods inside test classes.
18. Test method can not be used to test web-service call out . Please use call out mock .
19. You can't  send email from test method.
20.User, profile, organization, AsyncApexjob, Corntrigger, RecordType, ApexClass, ApexComponent ,ApexPage we can access without (seeAllData=true) .
21. SeeAllData=true will not work for API 23 version eailer .
22. Accessing static resource test records in test class e,g List<Account> accList=Test.loadData(Account,SobjectType,'ResourceName').
23. Create TestFactory class with @isTest annotation to exclude from organization code size limit .
24. @testSetup to create test records once in a method  and use in every test method in the test class .
25. We can run unit test by using Salesforce Standard UI,Force.com IDE ,Console ,API.
26. Maximum number of test classes run per 24 hour of period is  not grater of 500 or 10 multiplication of test classes of your organization.
27. As apex runs in system mode so the permission and record sharing are not taken into account . So we need to use system.runAs to enforce record sharing .
28. System.runAs will not enforce user permission or field level permission .
29. Every test to runAs count against the total number of DML issued in the process .