tests: add urlapi unittest

This adds a new unittest intended to cover the internal functions in
the urlapi code, starting with parse_port(). In order to avoid name
collisions in debug builds, parse_port() is renamed Curl_parse_port()
since it will be exported.

Reviewed-by: Daniel Stenberg <daniel@haxx.se>
Reviewed-by: Marcel Raad <Marcel.Raad@teamviewer.com>