Fix race condition in EndToEndTest.AssignsTransportSequenceNumbers

Don't verify increasing sequence numbers after test complesion as this
can be racy with regards to test shutting down send transports.

BUG=
R=stefan@webrtc.org

Review URL: https://codereview.webrtc.org/1269743004 .

Cr-Commit-Position: refs/heads/master@{#9672}
1 file changed