Add CreateSend/ReceiveTransport() methods to CallTest.

This allows the test to create its own transports if it, for instance, needs to do demuxing.

BUG=webrtc:5416

Review URL: https://codereview.webrtc.org/1573453002

Cr-Original-Commit-Position: refs/heads/master@{#11187}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: e74eef19bd3f101208dc72b98038e42fc523a351
7 files changed