Switch compiletests to `ui_test`

[Firestar99]

Switch from compiletest_rs to the more modern ui_test. Haven't looked at what it improves on, but seems to have parallel test runs.

[Firestar99]

ui_test also has differential test support, though it's not really documented:

https://github.com/oli-obk/ui_test/blob/main/examples/rustc_twice_with_different_flags.rs