Upgrade test suite to ui_test

[fpoli]

We should upgrade the test suite, replacing compiletest-rs with the new and less error-prone ui_test crate. A good example to start from is here.

While doing so, I believe that we should

• Merge all verify/verify_overflow/... folders into one, adding //@compile-flags: ... headers where needed. The separate folders were originally needed only because Prusti didn't support the -P... configuration flags that we now have.
• Merge all ui/pass/fail folders into one, adding //@check-pass and //@run headers where needed. This makes it easier to group and maintain related tests, because we no longer have to duplicate the folder structure under both pass and fail.

[fpoli]

ui_test might also be used to test the crates in prusti-tests/tests/cargo_verify/, replacing cargotest.rs. We should check.

[fpoli]

This is blocked by oli-obk/ui_test#147, because we need to test on Windows.