Fix auth-source-pass to search for hostname:port/username

Fix auth-source-pass to search for hostname:port/username

auth-source-pass supports entries with username either prefixed to the
hostname with an @ as separator or in a subdirectory under the
hostname.  This was true when there was no port or service included in
the name, but got broken with the introduction of
auth-source-pass-port-separator.

* lisp/auth-source-pass.el (auth-source-pass--find-match-unambiguous): Fix
to match hostname:port/username.
* test/lisp/auth-source-pass-tests.el: Add corresponding tests.