Fix Tramp IPv6 handling in tests

Fix Tramp IPv6 handling in tests

* lisp/net/tramp-gvfs.el (tramp-gvfs-maybe-open-connection):
* lisp/net/tramp-sh.el (tramp-maybe-open-connection): Improve message.

* lisp/net/tramp-integration.el (shortdoc): Add further examples of
`file-remote-p'.

* lisp/net/tramp.el (tramp-handle-file-remote-p): Extend docstring.

* test/lisp/net/tramp-tests.el (tramp-test02-file-name-dissect)
(tramp-test02-file-name-dissect-simplified)
(tramp-test02-file-name-dissect-separate): Extend tests.
(tramp-test06-directory-file-name)
(tramp-test26-file-name-completion)
(tramp-test26-interactive-file-name-completion): Better handling
of IPv6 hosts.