Disable 3 broken ref tests so that we can start blocking on check-ref again
Enable ref tests on darwin
auto merge of #1565 : larsbergstrom/servo/ref-tests-enable, r=pcwalton

Ref tests should now be stable enough to put them back in the list of checked tests that we run at checkin. This will at least ensure ACID1 no longer breaks.

I had to disable three tests that have regressed since ref tests first became unstable. I will open separate issues for each of them if we agree to land this PR.