check/gst/gstbin.c: Fix test case: we can't rely on a fixed state change order when going from READY => PAUSED becaus...

Original commit message from CVS:
* check/gst/gstbin.c: (test_children_state_change_order_flagged_sink),
(test_children_state_change_order_semi_sink):
Fix test case: we can't rely on a fixed state change order when
going from READY => PAUSED because the sink might commit its
new state first when the first buffer created by the source
reaches the sink before the source has finished its change state.
(Test case still fails at times, see #316856, comment 5 onwards)