Test infrastructure: add an option to only rerun tests that have failed

[smarter]

scalac partest supports this with --failed.

[dwijnand]

This isn't a bug, but I think it's a good candidate for the spree because it doesn't require lots of prior compiler knowledge and having this implemented would help all compiler hackers.

For context scala 2's partest leaves t1234.log files and uses that to determine failed tests. I think Scala 3's test framework uses i1234.out files.

[som-snytt]

Thanks, I needed this recently because I'm still bad at running dotty tests. The label should be prio:blocker!