rtph263ppay should accept any input compatible with its sink template
caps if it just outputs to e.g. udpsink or fakesink.

rtph263ppay ! rtph263pdepay should also work with any compatible input.
This would fail before with not-negotiated errors because the get_caps
function would see the encoding-name in the depayloader's template caps
and default to baseline H.263 because there's no profile/level information
in those caps, which is the right thing to do if downstream has filtercaps
from an SDP, but not if those fields are absent because they can be
anything like with the depayloader's template caps. Makes

  videotestsrc ! avenc_h263p ! rtph263ppay ! rtph263pdepay ! fakesink

work.