Import W3C CSS tests to validate CSS parsing (#12735)

Downloaded W3C conformance tests for CSS 2.1 and CSS 3 (selectors),
extracted all CSS (style tags, inline styles, and linked stylesheets),
Since each CSS is valid SCSS, the parser should accept them. Since these
are browser tests, some include CSS fragments which are intentionally
malformed. These are excluded via explicit exclusion lists.

Change-Id: Ib3347e1c559908f3bdd12817e55985e85f0009a2