Adding failing tests for using extend within a mixin when utilising variables

The recent release of stylus@0.30.1 has broken compatibility with 0.30.0.

I haven't been able to fix the problem, however this pull request is demonstrates failing tests that passed on 0.30.0.

I haven't been able to track down the commit what has caused this issue, however https://github.com/LearnBoost/stylus/commit/a6527b3cb6bbaafd740ebf9f739b3a684bb0e1eb seems like a possible candidate.

Let me know anyone needs any more information.