Shape margin parsing tests

This is branched from the parsing-test-additions branch in #520, so you only need to look at the commits on April 28 here and probably should wait to merge this one until 520 is merged.  This shouldn't be that much to review.

Also, the viewport units failures in 520 also fail for shape-margin, so I added a comment to Webkit bug #124502.  This suite also has some calc failures - logged Webkit bug #132313 and Blink bug #368069.