[TextBlob] Add a virtual test suite.

Add a new virtual test suite for running tests under fast/text and
svg/text with --enable-text-blobs.

R=jbroman@chromium.org,eae@chromium.org

Review URL: https://codereview.chromium.org/662473002

git-svn-id: svn://svn.chromium.org/blink/trunk@184023 bbb929c8-8fbe-4397-9dbb-9b2b20218538