[CSS Regions] Selection layout tests moved to specific folder

Patch already landed in WebKit. See http://trac.webkit.org/changeset/156796

Moved CSS Regions layout tests related to selection to "fast/regions/selection/".

Review URL: https://codereview.chromium.org/25868002

git-svn-id: svn://svn.chromium.org/blink/trunk@158855 bbb929c8-8fbe-4397-9dbb-9b2b20218538