test(firefox): introduce vendor-specific specs (#3890)

Certain Puppeteer methods do expose the inner browser - e.g.
`browser.version()` depends on the browser we run.

Split out these tests into a vendor-specific test suites.

References #3889

test/firefoxonly.spec.js

@@ -0,0 +1,39 @@
+/**
+ * Copyright 2019 Google Inc. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+module.exports.addTests = function({testRunner, expect, product}) {
+  const {describe, xdescribe, fdescribe} = testRunner;
+  const {it, fit, xit} = testRunner;
+  const {beforeAll, beforeEach, afterAll, afterEach} = testRunner;
+
+  describe('Firefox-specific tests', function() {
+    describe('Browser.version', function() {
+      it('should return whether we are in headless', async({browser}) => {
+        const version = await browser.version();
+        expect(version.length).toBeGreaterThan(0);
+        expect(version.startsWith('Firefox/')).toBe(true);
+      });
+    });
+
+    describe('Browser.userAgent', function() {
+      it('should include WebKit', async({browser}) => {
+        const userAgent = await browser.userAgent();
+        expect(userAgent.length).toBeGreaterThan(0);
+        expect(userAgent).toContain('Gecko');
+      });
+    });
+  });
+};
+