[css-flexbox] Fix for flex-margin-no-collapse.html

As described in
https://lists.w3.org/Archives/Public/www-style/2015Mar/0463.html, the
testcase is broken. The attached patch fixes it.

Also, could someone tell me whether https://hg.csswg.org/test or
https://github.com/w3c/csswg-test/blob/master/css-flexbox-1 is the
repository to use? Am I supposed to use github pull requests instead?

Thanks!

-christian