Support for non-reference tests

[hrj]

There are some tests in the CSS2.1 testsuite which don't have a reference. Also, there are some tests which I want to author myself, which won't have a reference, because they are intentionally complex.

Such tests can be effectively run as baseline tests. That is, we capture their screenshot and store the screenshot in the baseline directory. Then, we verify that the screenshot hasn't changed in a new version of the browser.

So, the mechanism will be very similar to #4, but the intention is different.