Add SASS unit tests

[james-nash]

Is your feature request related to a problem? Please describe.
As we add more SASS functions and mixins to our public API, we ought to test them to prevent bugs and regressions creeping in.

Describe the solution you'd like
It turns out there's a library called sass-true, for creating SASS unit tests. Looks like a good fit, maybe use it?

Describe alternatives you've considered
Not sure if any alternatives to sass-true exist. By all means have a look around and, if there are some, compare and pick the most appropriate one for Gravity.