Make FlexboxMixin tests more robust. #393

The FlexboxMixin tests fail sometimes on Saucelabs when a test runs before
the rules of the dynamically included CSS file are in effect. This was
fixed by including the CSS file statically with a reference tag instead of
dynamically.

A couple of other changes were made:
- Added support for utilizing LESS files with the reference tag.
- Fixed a bug which caused TestData to be copied to unit tests that didn't
  reference it.