tests/check/elements/: Fix the dp tests, but activating the pads for the streamheader tests too and cleaning up condi...

Original commit message from CVS:
* tests/check/elements/gdpdepay.c: (cleanup_gdpdepay),
(setup_gdpdepay_streamheader):
* tests/check/elements/gdppay.c: (cleanup_gdppay),
(setup_gdppay_streamheader):
Fix the dp tests, but activating the pads for the streamheader tests
too and cleaning up conditionaly