compiletest "--bless" fails to delete stderr files

[RalfJung]

I made a mistake during compiler development so that a "--bless" run (./x test ui --bless -- feat) ended up generating a bunch of stderr files for compiler runs that shouldn't fail. I fixed that mistake, but unfortunately "--bless" does not remove these files. It seems I have to delete them all by hand.

That's not great; if "--bless" creates new files then it should also clean them up by itself.