Fix Freeseer#647 Skip tests related to rmtp plugin for Windows

Partly fix Freeseer#647.
The rmtp plugin is not included in GStreamer for Win32, so the test based on it can cause problems.
This fix will skip these tests.

src/freeseer/tests/framework/test_plugins.py

@@ -79,6 +79,7 @@ def plugin_platform_category_cache():
     return plugin_platform_category_count
 
 
+@pytest.mark.skipif(sys.platform == 'win32', reason = 'Win32 GStreamer does not support rmtp plugin')
 @pytest.mark.parametrize('plugin_info, expected_instance', [
     (('AudioInput', 'get_audioinput_bin'), gst.Bin),
     (('AudioMixer', 'get_audiomixer_bin'), gst.Bin),