Add test for Tramp password handling

Add test for Tramp password handling

* lisp/net/tramp.el (tramp-error-show-message-timeout): New defvar.
(tramp-error-with-buffer, tramp-user-error): Use it.

* test/lisp/net/tramp-tests.el (tramp-error-show-message-timeout):
Set it to nil.
(tramp-test46-read-password): New test.
(tramp-test47-auto-load, tramp-test47-delay-load)
(tramp-test47-recursive-load, tramp-test47-remote-load-path)
(tramp-test48-unload):
* test/lisp/net/tramp-archive-tests.el (tramp-archive-test47-auto-load)
(tramp-archive-test47-delay-load): Rename.