tidy: find unnecessary `.stderr` files for `error-pattern` UI tests

[JohnTitor]

It could happen that someone is initially supposed to add a UI test using the //~ ERROR but noticed it's hard/impossible with that test, then used the error-pattern annotation instead.
In this situation, a .stderr file is created after running x.py test, but tidy won't complain about removing it after switching to error-pattern. Note that error-pattern doesn't need that file.
Yes, I made the exact same mistake in #97075, it'd be helpful if tidy finds it and tells us.

[JohnTitor]

Turns out this isn't true and something on my local is wrong, closing.