Should wait state change complete before start another state change.
Can't ensure can received async-done message when state change from PLAYING to PAUSED.
https://bugzilla.gnome.org/show_bug.cgi?id=736655
Test that a pipeline can change from PLAYING to PAUSED and back in
the following scenarios:
1. One track reach EOS after pushed some buffers while another track
still pushes buffers
2. One track reach EOS without buffers while another track still pushes
buffers
https://bugzilla.gnome.org/show_bug.cgi?id=736655