The test sent a media packet, then verified it was sent by checking the
"last packet sent"'s ID. But the last packet sent may have been
a STUN packet that came *after* the media packet.
BUG=webrtc:5978
Review-Url: https://codereview.webrtc.org/2071573002
Cr-Commit-Position: refs/heads/master@{#13156}