"System.AssertException: Assertion Failed" error in my test class

Hello, there! I've checked all similar posts but still wasn't able to solve the problem. This is a pretty simple trigger from Trailhead:

trigger RestrictContactByName on Contact (before insert, before update) {
	//check contacts prior to insert or update for invalid data
	For (Contact c : Trigger.New) {
		if(c.LastName == 'INVALIDNAME') {	//invalidname is invalid
			c.AddError('The Last Name "'+c.LastName+'" is not allowed for DML.');
		}
	}
}

This is my test class. The test class can get me 100% code coverage and the UI operation all works beautifully. The issue is the test result returned "System.AssertException: Assertion Failed" error at Line 15. Can anyone please explain to me why this is happening? Is it about Database opertion or DML statements? Thank you so much!

@isTest
private class TestRestrictContactByName {
     @isTest static void TestLastNameIsvalid() {	
        // Test data setup
        // Create a contact with a valid Last Name, and then try to insert/update it.
 		Contact c = new Contact (FirstName='Kathy',LastName='Smith',Department='Technology');
        insert c;
        // Perform test
        Test.startTest();
        Database.SaveResult result = Database.insert(c, false);
        Test.stopTest();
        // Verify 
        // In this case the insert/update should have been processed.
        System.assert(result.isSuccess());
    }          
    @isTest static void TestLastNameIsNotInvalid() {
        // Test data setup
        // Create a contact with a Last Name of 'INVALIDNAME', and then try to insert/update it.
        Contact c2 = new Contact (FirstName='Joe',LastName='INVALIDNAME',Department='Finance');
        insert c2;
        // Perform test
        Test.startTest();
        Database.SaveResult result = Database.insert(c2, false);
        Test.stopTest();
        // Verify 
        // In this case the deletion should have been stopped by the trigger,
        // so verify that we got back an error.
        System.assert(!result.isSuccess());
        System.assert(result.getErrors().size() > 0);
        System.assertEquals('The Last Name "INVALIDNAME" is not allowed for DML.',
                             result.getErrors()[0].getMessage());
    }   
}