Make RelayServerTest use VirtualSocketServer.
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

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 53cb74107f9ae07b0e2d04dd41d29248f55cec56

sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc,sippet/webrtc