Moves xmllite's unittests to rtc_unittest.

BUG=3836
R=andrew@webrtc.org, kjellander@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/26669005

git-svn-id: http://webrtc.googlecode.com/svn@7369 4adac7df-926f-26a2-2b94-8c16560cd09d