Using fake clock for TURN port tests and un-disabling some tests.

The fake clock has a few advantages:
1. It lets use verify that operations take the expected number of
   round trips.
2. It makes the tests faster by letting us remove the equivalent
   of "Sleep(500)" all over the tests.
3. It makes the tests less flaky, because sometimes sleeping for
   500ms or waiting for 1s is not enough.

R=honghaiz@webrtc.org, pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/2097793003 .

Cr-Original-Commit-Position: refs/heads/master@{#13304}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 716d07a2414389638eba6ed03b704bf752a269ef
4 files changed