Add a check to all.gyp to respect the include_tests variable.

When include_tests==0, tests should be excluded from the build. This
ensures libjingle_tests.gyp is excluded appropriately.

BUG=b/15673188
R=tnakamura@webrtc.org, wu@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/16729005

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6469 4adac7df-926f-26a2-2b94-8c16560cd09d