Some minor Tramp fixes, resulting from test campaign

Some minor Tramp fixes, resulting from test campaign

* lisp/net/tramp.el (tramp-handle-write-region):
* lisp/net/tramp-adb.el (tramp-adb-handle-write-region):
* lisp/net/tramp-sh.el (tramp-sh-handle-write-region):
* lisp/net/tramp-smb.el (tramp-smb-handle-write-region): Use `current-time'
if needed.

* lisp/net/tramp-gvfs.el (tramp-gvfs-gio-mapping):
(tramp-gvfs-do-copy-or-rename-file): Remove "gvfs-rename", it is
not trustworthy.

* test/lisp/net/tramp-tests.el (tramp-test07-file-exists-p): Check also for
symlinked files in trash.
(tramp-test20-file-modes): Revert last change, it was a thinko.