check/: fix tests for the new warning

check/: fix tests for the new warning

Original commit message from CVS:

* check/gst/gstbin.c:
* check/states/sinks.c:
fix tests for the new warning
* check/gst/gstpipeline.c:
add a test for pipeline and bus interaction
* gst/gstelement.c:
elements should be NULL if they get disposed; add a warning if not