New positioning model: Simple shape-margin tests

Update simple shape-margin tests to work with new positioning model.
Also move them to the CSSWG's repo.

BUG=260357

Review URL: https://chromiumcodereview.appspot.com/22276002

git-svn-id: svn://svn.chromium.org/blink/trunk@155554 bbb929c8-8fbe-4397-9dbb-9b2b20218538