There is really a lot to verify if one wants to do a complete test.
The entire codebase should be understood ideally.
Since some options depend on other options, one ought to test most combinations of settings.
Regressions with the persisting of settings should be tested. For example entering a non-lobbied MP game, disabling locked teams, then opening a lobby MP gamesetup and checking that teams are locked.
Test that all maptypes and the random map choice work as expected.
Test changing the number of players.