tests: Add a GFileMonitor test for G_FILE_MONITOR_WATCH_HARD_LINKS

Add a test for monitoring an existing local file, with the
WATCH_HARD_LINKS flag specified. This would previously cause a crash;
now it doesn’t.

This test contains a FIXME where I suspect we should be getting some
additional file change notifications from changes made through the hard
link; this requires further follow up and probably further fixes to our
inotify backend.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=755721