Handle `write-region' messages in Tramp properly

Handle `write-region' messages in Tramp properly

* lisp/net/tramp.el (tramp-handle-write-region-message): New defsubst.
* lisp/net/tramp-adb.el (tramp-adb-handle-write-region):
* lisp/net/tramp-gvfs.el (tramp-gvfs-handle-write-region):
* lisp/net/tramp-sh.el (tramp-sh-handle-write-region):
* lisp/net/tramp-smb.el (tramp-smb-handle-write-region): Use it.

* lisp/net/tramp.el (tramp-password-prompt-regexp)
(tramp-completion-mode-p):
* lisp/net/tramp-cmds.el (tramp-reporter-dump-variable)
(tramp-append-tramp-buffers):
* lisp/net/tramp-smb.el (tramp-smb-maybe-open-connection):
Use `bound-and-true-p'.

* lisp/net/tramp-compat.el (tramp-compat-delete-file):
Don't check for `boundp' anymore.

* test/lisp/net/tramp-tests.el (ert-x): Require it.
(tramp--test-messages): New defvar.
(tramp-test10-write-region): Extend test.