test: rtpbin_buffer_list: add test to verify that stats are correct

test: rtpbin_buffer_list: add test to verify that stats are correct

Add a test to verify that stats about sent and received packets are
correct even when using buffer lists.

NOTE: the newly introduced get_session_source_stats() selects the
desired source (sender or receiver) by filtering them by type (using the
get_sender parameter) rather than by ssrc because this simplifies the
code and it's good enough for testing purposes as there is usually one
source per type in the test setup.

Filtering by ssrc would have required handling asynchronous signals like
"on-new-sender-ssrc", with the relative locking, just to retrieve the
actual ssrc of the sender.