Fix race condition in EndToEndTest.AssignsTransportSequenceNumbers
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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#9672}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 8d62971611bac3e19d4489427a4b8346ad1e4ca8

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