tests: Add a test for the CSS parser

tests: Add a test for the CSS parser

Tests in the parser need 1 or 2 files:
1) test.css
2) test.ref.css (optional, defaults to test.css)

The test instantiates a CSS provider, loads test.css, then dumps the
loaded file to test.out.css and then checks that that file matches
test.ref.css. If not, it dumps a diff between those two to the log and
fails.

You want to run the test with --verbose to get the output dumped to
stdout.