SCSS testing

Created by: MartijnCuppens

Now that our flexibility of our scss is going to extended with PRs like https://github.com/twbs/bootstrap/pull/28445, we should have a look at automated tests for scss. With these tests I mean: check the output if scss configuration is changed (like adding utilities, etc..)

I used MochaJS for doing these tests with RFS, that worked pretty well, but maybe we can use the same "thing" for js and (s)css tests.