Don't skip test_that_video_index_is_written_on_eos on OS X

The GStreamer 1.0 webmmux (from macports gstreamer1-gst-plugins-good
1.2.3 package) does work correctly on OS X.
(FAILING TEST) Don't skip test_save_video on OS X

The GStreamer 1.0 webmmux does work correctly on OS X (see the previous
commit). However, test_save_video hangs indefinitely. I need to
investigate, but in the meantime I shouldn't hide the problem from
myself (or anyone else testing on OS X).