CI: Add tests for controller.game

[cyberrumor]

controller.game has been accumulating a lot of functionality lately, but it's not automatically tested anywhere.

To test it, we can probably manipulate the environment to say HOME is /tmp/home, and make some context managers that create and destroy the folders we want to test against before/after each test.

• Test no supported games installed
• Test library.vdf parsing
• Test one or multiple steam games installed
• Test one or multiple flatpak games installed
• Test unique games installed in flatpak and steam
• Test the same game is installed through both flatpak and steam
• Test that UI(ammo_controller).controller attributes are created the way we expect.
• Test that arguments like --conf, --downloads and --mods behave correctly.