tests: rtpgstpay/-depay: add new unit test

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719587
rtpgstpay: delay event until next buffer, to get a timestamp

Previously events, e.g. tag or custom events, would be flushed out of
rtpgstpay's internal adapter before the first buffer arrived. The events
are not timestamped and so the output buffers containing the RTP packets
would not be timestamped either. Now, these events are delayed until the
first buffer has arrived. Moreover they will get the same timestamp as
the first buffer.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=719587