Part of the `ScreenFooter` refactor, which intends to move the footer
content handling to `OsuScreen`. This commit updates the `ScreenFooter`
test to operate on entire `OsuScreen`s, in order to better test the
entire flow of pushing a screen, and having it create and add its own
content to the footer.
This should be 80-90% identical to the original test case structure
wise, just that instead of manually manipulating the footer with
`SetButtons()`, various screens with the appropriate buttons are being
moved around the screen stack.
Additionally this adds some more tests handling common use cases, as
well as removes `TestLoadOverlayAfterFooterIsDisplayed()`, since as far
as I understand the behaviour described in it doesn't actually happen in
production code. From what I can see, Screens instantiate their overlays
in `load()`, and then register them in `LoadComplete()`. There seems to
be no case where a `ShearedOverlayContainer` is created in the middle of
a screen's lifecycle.