[css-regions] New CSS Regions tests

Hi there,

I've added a couple of Regions tests around animations[1], canvas/iframes/video[2], positioning[3] and 3D transforms[4]. These hit current bugs in the WebKit/Blink compositing model, but should be usable as general tests, too.
Reviews will be deeply appreciated :)

Thanks a lot,
Mihai

[1] http://hg.csswg.org/test/rev/132e2403d98c
[2] http://hg.csswg.org/test/rev/6659e8393b12
[3] http://hg.csswg.org/test/rev/2afdc77698d3
[4] http://hg.csswg.org/test/rev/ec8651454b84