The second assert fails, and seems like the opportunity owner is not changed at all, how can i simulate the account owner change like they do in the UI? (Whenever an account owner is changed, all the opportunities owners are changed to the same one, didnt happen here)

The second assert fails, and seems like the opportunity owner is not changed at all. in the UI - When the account records transferred to another user, the associated open opportunities automatically transferred to the new account owner without firing the opportunity trigger.

(Whenever an account owner is changed, all the opportunities owners are changed to the same one, didnt happen here)

how can i simulate the same process in a test class?

The second assert fails, and seems like the opportunity owner is not changed back, and the future is not running. i dont really know how to run this future method (looked at SF documentation).

The second assert fails, and seems like the opportunity owner is not changed at all, how can i simulate the account owner change like they do in the UI? (Whenever an account owner is changed, all the opportunities owners are changed to the same one, didnt happen here)

that's the test i wrote:

@isTest private static void testavoidOpportunityOwnerChangeOnOwnerChange()
    {
        Account accountTest = TestUtil.getAccountByName('AccountTestAvoid');
        insert accountTest; 
        
        Opportunity testOpp = TestUtil.getOpportunityByNameAndStage('TestOne', 'Customer Confirmed Quote', accountTest.id);
        testOpp.type = 'Renewal Business';
        insert testOpp;
        
        Account acc = [SELECT Id, Name,OwnerID from Account Where name = 'AccountTestAvoid' limit 1];
        Opportunity opp = [SELECT id,name,ownerId from opportunity limit 1];
        
        Profile p = [SELECT Id FROM Profile where name ='system administrator' limit 1]; 
        User usr = TestUtil.getUserByNameAndProfileId('test',p.Id);
        insert usr ;
        
        accountTest.ownerId = usr.id;
        update accountTest;
     
        Account accRes = [SELECT Id, Name,OwnerID from Account Where name = 'AccountTestAvoid' limit 1];
        Opportunity oppRes = [SELECT id,name,ownerId from opportunity limit 1];
        
        
        System.AssertNotEquals(accRes.Id,acc.Id);
        System.AssertNotEquals(oppRes.Id,opp.Id);
    }

that's the test i wrote: (UPDATED AFTER YOUR SUGGESTIONS)

@isTest private static void testavoidOpportunityOwnerChangeOnOwnerChange()
    {
        Account accountTest = TestUtil.getAccountByName('AccountTestAvoid');
        insert accountTest; 
        
        Opportunity testOpp = TestUtil.getOpportunityByNameAndStage('TestOne', 'Customer Confirmed Quote', accountTest.id);
        testOpp.type = 'Renewal Business';
        insert testOpp;
        
        Account acc = [SELECT Id, Name,OwnerID from Account Where name = 'AccountTestAvoid' limit 1];
        Opportunity opp = [SELECT id,name,ownerId from opportunity limit 1];
        
        Profile p = [SELECT Id FROM Profile where name ='system administrator' limit 1]; 
        User usr = TestUtil.getUserByNameAndProfileId('test',p.Id);
        insert usr ;
        
        accountTest.ownerId = usr.id;
        
        Test.startTest();
         update accountTest;
        Test.stopTest();
     
        Account accRes = [SELECT Id, Name,OwnerID from Account Where name = 'AccountTestAvoid' limit 1];
        Opportunity oppRes = [SELECT id,name,ownerId from opportunity limit 1];
        
        System.debug('accRes.OwnerId: ' + accRes.OwnerId);
        System.debug('acc.OwnerId: ' + acc.OwnerId);
        System.debug('oppRes.OwnerId: ' + oppRes.OwnerId);
        System.debug('opp.OwnerId: ' + opp.OwnerId);
        System.AssertNotEquals(accRes.OwnerId,acc.OwnerId);
        System.AssertNotEquals(oppRes.OwnerId,opp.OwnerId);
    }