rtc_unittest: prevent execution of broken tests.

BUG=1976
R=kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/29509004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7193 4adac7df-926f-26a2-2b94-8c16560cd09d