Unit tests for LESS/SCSS

Also fixes for #8894 and #8895.
Seems like there are many more combinations of selectors (http://www.w3.org/TR/selectors/) that could be added to test the parsing code:

* different combinations of tag, class, id, attribute, universal, pseudo-classes, and pseudo-elements
* selector combinators: `>`, `+`, `~`
* `@rules`
* selector lists
* multiple levels