Fix world closing in client gametests

[Earthcomputer]

There was an issue where the assertion that the game is on the title screen would fail when ending a test by closing a singleplayer world (via a try-with-resources close of TestSingleplayerContext).

This was caused by the threading system deferring the client.disconnect call until after the client thread task wait loop (i.e. until the gametest thread waits a tick). Therefore, the order of operations inside GameMenuScreen.disconnect was reversed and the title screen was shown first before disconnecting. Moreover, the title screen was visible to the gametest thread until ticks were waited, which allowed context.waitForScreen(TitleScreen.class) to return immediately after waiting 0 ticks.

Rather than relying on navigating the game menu which may have been changed by mods, I thought it was better to refactor the code to disconnect directly, and I've written it in a way which waits for the world to be null directly after calling client.disconnect, which allows for the deferred call to run.

[kevinthegreat1]

Works for me. (Tested by manually using this version.)