fix a race condition in test_buffer.py

fix a race condition in test_buffer.py

Original commit message from CVS:
fix a race condition in test_buffer.py

* gst/gst.override:
* gst/gstmodule.c: (init_gst):
add a pygst debug category for bindings themselves to use
* gst/gstbuffer.override:
add a repr method; add some assertions
* gst/pygstminiobject.c: (pygst_miniobject_init),
(pygstminiobject_register_wrapper), (pygstminiobject_new),
(pygstminiobject_new_noref), (pygstminiobject_dealloc),
(pygstminiobject_clear):
make the miniobjs hash private with an underscore
add debugging for inserting/removal in hash
fix pygstminiobject_clear - it also needs to remove
from the global hash.  Fixes a nasty race problem in
test_buffer
* testsuite/test_buffer.py:
expand on the subbuffer test