Silence tests unsupported in PhantomJS.

These tests can still be run in the browser using `grunt test --debug`.

This is a repeat of 42f8d155f8f240c457c969ac683d8b25876708ee. For posterity, we
do this because Phantom has a problem with Object.freeze and the test runner
can't do __DEV__ right (because we get rid of that in the build step).