Sass unit tests

Having unit tests for our Sass functions is probably a good idea. It certainly may help prevent something like [this](https://github.com/zurb/foundation/issues/5756) from happening again.

The only unfortunate thing is that there aren't currently any test frameworks that work with libsass, and arguably the most useful thing we could be testing is compatibility between Ruby Sass and libsass. That's just something else we'll need to figure out with our deploy process—is there a way to detect that two stylesheets compiled with different libraries are identical?

**Functions that need tests**
 - [ ] `map-serialize`
 - [ ] `map-next`
 - [x] `isitlight`
 - [x] `smartscale`
 - [x] `hasvalue`
 - [ ] `get-side`
 - [ ] `get-border-value`
 - [ ] `get-shadow-value`
 - [ ] `strip-unit`
 - [ ] `convert-to-rem`
 - [ ] `rem-calc`