Move the linux subpixel tests to a virtual test suite and re-enable

BUG=243378
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/27711007

git-svn-id: svn://svn.chromium.org/blink/trunk@159928 bbb929c8-8fbe-4397-9dbb-9b2b20218538