Fix broken compass test in ScssphpFilter

The scss lib throws an exception if you reference an undefined mixin,
which is what this test does. I've split it into two tests - one
checking that the mixin works when compass is enabled, and the other
checking that it throws an exception when the mixin is referenced and
compass is not enabled.
Merge pull request #594 from ciarand/feature/scssphp-fix-test

Fix broken compass test in ScssphpFilter