Make RelayServerTest use VirtualSocketServer.

Permits running the tests in parallel.

R=juberti@webrtc.org
BUG=2597
TEST=third_party/gtest-parallel/gtest-parallel -w64 out/Debug/rtc_unittests --gtest_filter=RelayServerTest.*

Review URL: https://webrtc-codereview.appspot.com/38479004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7974 4adac7df-926f-26a2-2b94-8c16560cd09d