Convert fast/multicol/positioned-split to reftest.

 * Set explicit column-gap and line-height.
 * Clarified pass condition.
 * Removed useless evil twin in fast/multicol/newmulticol.

R=mstensho@opera.com
BUG=386567

Review URL: https://codereview.chromium.org/702093002

git-svn-id: svn://svn.chromium.org/blink/trunk@184967 bbb929c8-8fbe-4397-9dbb-9b2b20218538