Replaying FF mochitest-chrome: "Failed to stat .../libjsctypès-tëßt.so: replay is impossible"

The test appears to copy libjsctypes-test.so to this unicode path and then remove the unicode name. For some reason that temp file seems to be mmap'd by something. That file breaks rr's avoid-copying-on-mmap heuristics because it's read-only and executable and not on tmpfs, so removing it during recording breaks replay.

I would probably be OK punting this problem for 1.0. Another option is to make our avoid-copying-mmap'd-file heuristics less aggressive. They were tuned by optimizing FF startup time, but that's less important over minutes-long test suites. Depends on what we want to optimize for. Or maybe there's some other clever thing we can do (f.e. keep a record of mmap'd files that pass the don't-need-to-copy heuristic, and if one of them is unlink()d during recording, copy it).

Start by reproducing the Firefox mochitest-chrome replay failure and trace rr's avoid-copying-on-mmap heuristics around the removed libjsctypes-test.so file. Compare the recording and replay behavior when a read-only executable mmap'd file is unlinked. Done means the test can be recorded and replayed without the “Failed to stat” error.