Fix #647 Skip tests related to RTMP plugin for Windows

Partly fix #647.
The RTMP plugin is not included in GStreamer for Win32, so the test based on it can cause problems.
This fix will skip these tests.
Probably best to just add a simple conditional when iterating through the plugins like how is done for the Firewire issue, since using pytest's xfail or skipif do too much with the way this test is set up.