Improve file-notify-test08-watched-file-in-watched-dir

Improve file-notify-test08-watched-file-in-watched-dir

* test/automated/file-notify-tests.el (file-notify--test-desc2):
New variable.
(file-notify--test-cleanup): Use it.
(file-notify--test-event-handler): Enable trace.
(file-notify-test08-watched-file-in-watched-dir): Tag it as
:expensive-test.  Rewrite callbacks to use
`file-notify--test-event-handler'.  Read events in loop.  Check
`file-notify--test-results'.
(file-notify-test08-watched-file-in-watched-dir): Fix docstring.