Add more backends to the integration tests

[notgull]

On the it branch, I've added an X11 backend that runs the tests in a Docker container. We should add other backends as well to match winit's backends.

• For Wayland, we can probably just add a headless Wayland compositor to the above Docker image.
• For Web, we can compile to WASM and run in a headless browser. (cc @daxpedda, you said you wanted to take a crack at this)
• For Windows, we can either run it in Wine under headless X11 or figure out if there is a way to do Windows heedlessly.
• For Android and iOS, we can use their simulators.
• Not sure about macOS, @madsmtm any ideas?
• Not sure about Orbital, @jackpot51 any ideas?

See #2866 for top-level issue about integration testing.

[jackpot51]

I wouldn't recommend automated testing with Redox at this point, though it can be done with redoxer.

[madsmtm]

From what I know, there's basically no way for non-macOS systems to run tests on macOS/iOS, not even in a virtual machine.

If we restrict this to "testing those platforms only work on macOS", then I think either using the XCTest framework and integrating with Xcode, or using some external tool to monitor the external state of the window, perhaps using AppleScript, would be the way forwards? Though I haven't researched this nearly enough.