Users should not be deleted in tests that test user creation.
Tests should use unique user names to avoid collisions.
User deletion should use user_remove anyway.
PHPBB3-10972
Always add users, do not keep track of which users have been added.
The tests should know whether users they want exist or not.
Use more unique user names in tests for robustness.
Added some more assertions here and there.
PHPBB3-10972
