unit tests: error if there is a .stderr without matching .rs

[matthiaskrgr]

Right now we have a ./tests/ui/map_unit_fn.stderr but on matching ./tests/ui/map_unit_fn.rs which is going to be cleaned up by #3570 .

Maybe have some tests that assures that all .rs files have a matching .stderr?

[flip1995]

Maybe have some tests that assures that all .rs files have a matching .stderr?

I guess this is a typo, since you wrote it correctly in the title. But just to clarify: It should only warn on a *.std[err|out] file without a matching *.rs file. A *.rs file without a *.std[err|out] file is valid and means "No output expected".

[mati865]

Fixed by #3669