Use UITestFixture as it requires the overlay site to be set up

Author: sgiehl

tests/UI/specs/OptOutIframe_spec.js

@@ -8,8 +8,6 @@
  */
 
 describe("OptOutIframe", function () {
-    this.fixture = "Piwik\\Tests\\Fixtures\\OneVisit";
-
     const siteUrl = "/tests/resources/overlay-test-site-real/opt-out.php?implementation=iframe",
         safariUserAgent = "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_9_3) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/7.0.3 Safari/7046A194A",
         chromeUserAgent = "Mozilla/5.0 (Windows NT 6.1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/41.0.2228.0 Safari/537.36";