All tests are child classes of this class, so it doesn't make any difference if function is public or protected. Currently all other similar functions, such as create_user() are protected as well.

As for styles_manager or similar functionality, that's overkill. Installing styles isn't needed anywhere in phpBB, not even third party mods do that, so moving this to phpBB is a lot of unnecessary work. Then there is issue of these tests working with 3.0 and 3.1, which would mean keeping this code in tests for 3.0 anyway.