Fix Eshell handling of remote files like "/ssh:remote:~/file.txt"

Fix Eshell handling of remote files like "/ssh:remote:~/file.txt"

* lisp/eshell/em-glob.el (eshell-glob-convert): Use 'concat' instead of
'file-name-concat' to avoid extraneous slashes.
(eshell-extended-glob): Bail out if we didn't find a glob after all.

* test/lisp/eshell/em-glob-tests.el (tramp): Require.
(em-glob-test/convert/remote-start-directory): Use the mock remote
connection.
(em-glob-test/remote-user-directory): New test.

(cherry picked from commit 1529ad0315f8d4a96ca07969c1c91c1c50bb6075)