compiletest: previously-succeeding tests are still rerun.

[eddyb]

Running e.g. ./x.py test --stage 1 src/test/mir-opt several times currently results in all the tests being ran every single time, instead of skipping those that succeeded already.

I'm not sure when this started happening, but it's recent, somewhere in the past few weeks.
It could perhaps be caused by #56680, although I don't recall having issues around the time that PR was merged. Also, I have no indication that anyone else has this issue, could be my build dir.

cc @Mark-Simulacrum

[euclio]

See #57529 as well.

[oli-obk]

I can confirm this happening on my machine, too. I've only had this for a few days though (definitely not last week) and I rebase very frequently

[Mark-Simulacrum]

I will attempt to investigate this tonight and try and get a fix out soon, but not sure yet if I'll have the time.

[Mark-Simulacrum]

I've posted the fix at #57871