Core: Implements QUnit.skip

Fixes #637
Fixes #434
It doesn't.  That is done in the `QUnit.skip` method by setting the test's `callback` property to a dummy function instead of the function that may or may not have been passed into `QUnit.skip`.

https://github.com/jquery/qunit/pull/652/files#diff-7eb52b366866677666470e019283c8eaR139