Rename tests/ui/unique to `box`

[jyn514]

https://github.com/rust-lang/rust/tree/fe0b0428b89802f02a050eab72373b75709d0bce/tests/ui/unique all seem to be testing std::boxed::Box. It hasn't been called unique in many years, and that's now ambiguous with ptr::Unqiue. We should rename the test suite for clarity.

[reez12g]

@rustbot claim

[Noratrieb]

Most of these should be unit tests in library/alloc, not UI tests. Although given the amount of compiler magic behind it it my make sense to keep them as UI tests.

[jyn514]

No objection to that, but renaming the directory doesn't make it any harder to switch them to unit tests in the future.