How to cover apexpages.currentpage().getparameters().get('id') in test class?

you can use this code....

@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();
}
}