Finish work on filenotify-tests.el

Finish work on filenotify-tests.el

* test/lisp/filenotify-tests.el (file-notify--test-monitors):
New variable.
(file-notify--test-cleanup, file-notify--test-monitor): Use it.
(file-notify--test-read-event, file-notify-test02-events)
(file-notify-test04-file-validity): Handle "gvfs-monitor-dir.exe".
(file-notify-test03-autorevert)
(file-notify-test08-watched-file-in-watched-dir):
Set `file-notify--test-desc' for proper work of
`file-notify--test-monitor'.  (Bug#21804)