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.
Won't this skip all output plugin tests instead of just the "RTMP Streaming" plugin?

I was thinking a conditional similar to the one below.
```python
for plugin in plugins
    if plugin.name == "RTMP Streaming" and sys.platform == 'win32':
        continue
```