Improve testfilemonitor test repeatability and debuggability

Merged Philip Withnall requested to merge pwithnall/glib:1634-testfilemonitor-race into master

This changes testfilemonitor to use a per-test temp directory so you can use it with --repeat in Meson, and it improves debug output on failure.

It doesn’t fix the test though, because I can’t get it to fail in the first place.

Helps #1634

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