tests/check/: Add add testsuite for the rtp-payloader that tries simulating dataflow. Needs more test data.

Original commit message from CVS:
* tests/check/Makefile.am:
* tests/check/elements/.cvsignore:
* tests/check/elements/rtp-payloading.c:
Add add testsuite for the rtp-payloader that tries simulating
dataflow. Needs more test data.