Factor out test configuration for remote files

Factor out test configuration for remote files

* lisp/emacs-lisp/ert-x.el (tramp-methods)
(tramp-default-host-alist): Declare.
(ert-remote-temporary-file-directory): New defconst.

* test/README: Mention dnd-tests.el.

* test/lisp/autorevert-tests.el
(auto-revert-test-remote-temporary-file-directory): Remove.
Replace all uses by `ert-remote-temporary-file-directory'.

* test/lisp/dnd-tests.el (ert-x): Require.
(dnd-tests-temporary-file-directory): Remove.  Replace all uses by
`ert-remote-temporary-file-directory'.

* test/lisp/filenotify-tests.el
(file-notify-test-remote-temporary-file-directory): Remove.
Replace all uses by `ert-remote-temporary-file-directory'.

* test/lisp/shadowfile-tests.el (ert-x): Require.
(shadow-test-remote-temporary-file-directory): Remove.  Replace
all uses by `ert-remote-temporary-file-directory'.

* test/lisp/net/tramp-tests.el: Don't require ert.
(ert-remote-temporary-file-directory): Define if it doesn't exist.
(tramp-test-temporary-file-directory): Remove.  Replace all uses
by `ert-remote-temporary-file-directory'.