Rework filenotify-tests.el

Rework filenotify-tests.el

* lisp/filenotify.el (file-notify--rm-descriptor):
Use `insert-special-event'.

* lisp/net/tramp-sh.el (tramp-bundle-read-file-names): Fix docstring.
(tramp-sh-gio-monitor-process-filter): Extend error strings.

* test/infra/Dockerfile.emba (emacs-base): Install python3.

* test/lisp/filenotify-tests.el (auto-revert-buffer-list): Declare.
(file-notify--test-rootdir): New defvar.  Replace all uses of
`temporary-file-directory' by it.
(file-notify--test-cleanup): Set `auto-revert-buffer-list' to nil.
(file-notify--test-monitor): Check also for `process-live-p'.
Skip test, if the monitor cannot be determined.
(file-notify--deftest-remote): Do not call
`tramp-cleanup-connection', this is done in
`file-notify--test-cleanup' already.
(with-file-notify-test): New defmacro.  Use it in (almost) all tests.
(file-notify--test-make-temp-name): Do not create
`file-notify--test-tmpdir'.
(file-notify--test-event-handler): Use `directory-file-name '.
(auto-revert-debug): Set it to nil.
(file-notify-test<nn>-*): Rewrite, using `with-file-notify-test'
and `ert-with-temp-file'.

* test/lisp/net/tramp-tests.el (tramp-test46-file-notifications):
Tag it :unstable.

(cherry picked from commit 76c17ac0f018dd860f0829e78f780bcd884466c6)