Don't create a top-level `true` directory when running UI tests
The funny thing about writing `-Cincremental=true` is that it *does* enable incremental compilation ... using an incremental compilation dir of `./true`.
And for UI tests, that ends up creating a `true` directory in the repository root, which is annoying.
Fortunately, compiletest has an existing `//@ incremental` directive that takes care of creating an empty incremental directory, and passing it to `-Cincremental`.
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I have manually checked that reverting rust-lang/rust#146649 still causes the updated test to fail.
de20efd01e