gst/gstbin.c: Fix race for bins that simulate ASYNC state changes by inserting

gst/gstbin.c: Fix race for bins that simulate ASYNC state changes by inserting

Original commit message from CVS:
* gst/gstbin.c: (gst_bin_add_func), (gst_bin_remove_func),
(gst_bin_change_state_func), (bin_handle_async_done),
(gst_bin_handle_message_func):
Fix race for bins that simulate ASYNC state changes by inserting
ASYNC_START and ASYNC_DONE messages in their bus. We need to check for
pending ASYNC messages even when the bin does not have ASYNC children.
We note detect this behaviour because we will receive an ASYNC message
that is originating from the bin itself.
Fixes races with decodebin2 state changes.
* tests/check/gst/gstbin.c: (GST_START_TEST):
Add some more debug.