I have ControllerExtension for Account’s child Object. Obj1   Profile A  User UA has permission to CRUD for Account and OBJ1. Profile B  - User UB doesn’t have access to ControllerExtension as well as VF page.  When I login as user UB in developer sandbox, and test through UI, I get insufficient privileges for page using  ControllerExtension. This is as expected.
I have written test class to test this scenario. But it doesn’t give me this error.

But if I use Chatter Free profile to create runAs User , I get the exception in catch part.
Is there way to catch or debug insufficient privilages?

How to write test class to test User Privileges?
@isTest
    public static void testUserAccess(){
        // create test user
         User runUsr =  [select name, id, profileId from User where id = '005j000034BMsou' limit 1];
        //TestExtension.createProfileBUser();
//As this doesn’t work, I am directly retrieving user who as profileB
        System.debug(' runUsr ' + runUsr);
   // This prints me correct user         
        System.runAs(runUsr){
            user rrU = [select name, id, profileId                             from user
                        where id = :UserInfo.getUserId()];
            System.debug('runUsr rru' + runUsr + ' ' + rrU); 
            System.assertEquals(runUsr,rrU) ;              
            //Both above users are matching
            
// Create Account        
            Account account = new Account(name='testAccount');
            insert account;
            // I expect error here but no error
            // Create the controllerExtension and click cancel
            ApexPages.StandardController std =  new ApexPages.StandardController(account);
            CCExtension ctrl;
            PageReference page;
            try{
                ctrl = new CCExtension (std);
                System.debug(' Created ctrl ' + ctrl );
                System.AssertEquals(null,ctrl);
                page = ctrl.save();
            } catch(Exception e){
                System.debug(' CTRL creation Exception e ' + e);
// No error message here too
            }
             System.assertEquals(null, page);
// this passes - is this the way to write the test?
            if(ApexPages.hasMessages()){
                System.debug('There are error messages');
                for(ApexPages.Message msg:ApexPages.getMessages()){
                    System.assert(true, ' Error messages ' +  msg );
                }
            }
            
        }