Convert fast/multicol/margin-collapse to reftest.

The only notable change is setting column-gap explicitly.

R=mstensho@opera.com
BUG=386567

Review URL: https://codereview.chromium.org/696273002

git-svn-id: svn://svn.chromium.org/blink/trunk@184825 bbb929c8-8fbe-4397-9dbb-9b2b20218538