Don't use internal filenotify function in test

Don't use internal filenotify function in test

* test/lisp/filenotify-tests.el
(file-notify--test-file, file-notify--test-add-watch): New.
(file-notify--test-event-test, file-notify--test-event-handler)
(file-notify-test02-rm-watch, file-notify-test03-events)
(file-notify-test05-file-validity, file-notify-test07-many-events)
(file-notify-test08-backup, file-notify-test09-watched-file-in-watched-dir):
Avoid using the internal `file-notify--event-watched-file' so that it
can be removed from filenotify.el.
Instead, pass the file name to the callback as an extra argument;
use `file-notify--test-add-watch' instead of `file-notify-add-watch'.