Don't skip test_that_video_index_is_written_on_eos on OS X

GStreamer 1.0's 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

GStreamer 1.0's 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). test_save_video does pass on
Linux.