Allow enabling `-Zui-testing=yes`

On nightly, the compiler flag `-Zui-testing=yes` changes the diagnostic emitter to not print line numbers, instead replacing all of them with `LL`. This helps in two ways: when changing a comment at the beginning of a test file, fewer things in the render change, making it easier to review, but more importantly the length of the line number is always the same, so when any of the width boundaries change (9 -> 10, 99 -> 100, etc.), the stderr output doesn't dramatically change for the entire diagnostic purely because the code was moved around. There's also a change recently introduced in nightly which changes the alignment of the numbers from left to right, which will be annoying for users of trybuild when they update and could be mitigated with this.

Sadly, we can't use the env flag `RUSTFLAGS` to pass the flag ourselves to `rustc` because trybuild removes it.