feat(firefox): Page.waitForRequest/Page.waitForResponse (#3989)

Drive-by: refactor `Request.frame()` tests into a separate test suite.

test/utils.js

@@ -97,6 +97,10 @@ const utils = module.exports = {
     }
   },
 
+  isFavicon: function(request) {
+    return request.url().includes('favicon.ico');
+  },
+
   /**
    * @param {!Page} page
    * @param {string} frameId