Add tests for remote files in auto-revert-tests

Add tests for remote files in auto-revert-tests

* lisp/autorevert.el (auto-revert-debug): New defvar.
(auto-revert-notify-handler): Write traces.

* lisp/filenotify.el (file-notify-debug): New defvar.
(file-notify-handle-event, file-notify-callback): Write traces.

* lisp/net/tramp-sh.el (tramp-sh-handle-vc-registered):
Handle nil `vc-handled-backends'.

* test/lisp/autorevert-tests.el
(auto-revert-test-remote-temporary-file-directory): New defconst.
Handle also $REMOTE_FILE_NOTIFY_LIBRARY.
(auto-revert--test-enabled-remote-checked): New defvar.
(auto-revert--test-enabled-remote): New defun.
(auto-revert--wait-for-revert): Rewrite without timeout.
(auto-revert--deftest-remote): New defmacro.
(auto-revert-test01-auto-revert-several-files):
(auto-revert-test02-auto-revert-deleted-file): Adapt for remote files.
(auto-revert-test02-auto-revert-deleted-file):
Use `auto-revert-debug' for debug messages.
(auto-revert-test00-auto-revert-mode-remote)
(auto-revert-test01-auto-revert-several-files-mode-remote)
(auto-revert-test02-auto-revert-deleted-file-mode-remote)
(auto-revert-test03-auto-revert-tail-mode-mode-remote)
(auto-revert-test04-auto-revert-mode-dired-mode-remote): New tests.

* test/lisp/filenotify-tests.el (file-notify--test-event-handler):
Use `file-notify-debug' for debug messages.