Improve robustness of filenotify-tests.el (Bug#23618)

Improve robustness of filenotify-tests.el (Bug#23618)

* test/lisp/filenotify-tests.el (file-notify--test-no-descriptors)
(file-notify--test-no-descriptors-explainer)
(file-notify--test-cleanup-p): New defuns.
(file-notify--test-cleanup): Don't check for
`file-notify--test-event' anymore, that's done in
`file-notify--test-no-descriptors'.
(file-notify--test-with-events-check)
(file-notify--test-with-events): Handle the `random' marker.
(file-notify--test-with-events-explainer): Improve readability.
(file-notify-test00-availability)
(file-notify-test01-add-watch, file-notify-test02-events)
(file-notify-test03-autorevert)
(file-notify-test04-file-validity)
(file-notify-test05-dir-validity)
(file-notify-test06-many-events, file-notify-test07-backup)
(file-notify-test08-watched-file-in-watched-dir)
(file-notify-test09-sufficient-resources): Check also
`file-notify--test-cleanup-p'.
(file-notify-test04-file-validity)
(file-notify-test05-dir-validity): Use `ignore' as handler.
(file-notify-test05-dir-validity)
(file-notify-test06-many-events): Delete directory finally.
(file-notify-test08-watched-file-in-watched-dir): Add `random' marker.