Bug 1367636 - Enable stylo-vs-stylo reftest on CI. r=heycam

MozReview-Commit-ID: zWMi4iKLA6

taskcluster/ci/test/test-sets.yml

@@ -79,6 +79,7 @@ headless:
 stylo-tests:
     - cppunit
     - crashtest
+    - reftest
     - reftest-stylo
     - mochitest-style
     - mochitest-chrome-style