gstreamer-buffering: Pick up a running port by simple-httpd

Even if the port is already in use, simple-httpd doesn't report and
just show a message "Waiting for requests...". It's a simple bug of
example of libsoup2.4.
Fortunately, without a specific port, simple-httpd will pick up
a port which is available. Then, we can parse its stdout to parse
running port.

Signed-off-by: Justin Kim <justin.kim@collabora.com>
Reviewed-by: Mathieu Duponchelle <mathieu.duponchelle@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D4080