Support for more test suites

[hrj]

The css tests from w3c have many more tests, apart from the css21 tests that we are currently focussed on.

Some of the categories we are interested in are:

• css-color
• css-flexbox
• css-lists
• cssom
• selectors-3
• css-text
• css-align
• css-images3

Also, there are cookie tests, from w3c. Don't have the link right now. These tests are not automated and hence we were testing these manually before. These should now be automatable with the image comparison technique.

[hrj]

This repo contains all the built w3c css tests.