Handle the situation when a call to gst_rtsp_media_set_state is done
when media status is preparing.
Also add unit test for this scenario.
The unit test simulate on a media level when two clients share a (live)
Both clients have done SETUP and got responses. Now client 1 is doing
play and client 2 is just closing the connection.
Then without patch there are a problem when
client1 is calling gst_rtsp_media_unsuspend in handle_play_request.
And client2 is doing closing connection we can end up in a call
to gst_rtsp_media_set_state when
priv->status == GST_RTSP_MEDIA_STATUS_PREPARING and all the logic for
shut down media is jumped over .
With this patch and this scenario we wait until
priv->status == GST_RTSP_MEDIA_STATUS_PREPARED and then continue to
execute after that and now we will execute the logic for
shut down media.