sauce labs <3 mention in TESTING.md (#20442)

* sauce labs <3

* address comment

* update snippet with Joey's comments

contributing/TESTING.md

@@ -199,7 +199,7 @@ For testing documents on arbitrary URLs with your current local version of the A
 
 ## Testing on Sauce Labs
 
-In general local testing (i.e. `gulp test`) and the automatic test run on [Travis](https://travis-ci.org/ampproject/amphtml/pull_requests) that happens when you send a pull request are sufficient.  If you want to run your tests across multiple environments/browsers before sending your PR you can use Sauce Labs.
+We use [Sauce Labs](https://saucelabs.com) to perform cross-browser testing (thanks Sauce Labs!). In general local testing (i.e. gulp test) and the automatic test run on [Travis](https://travis-ci.org/ampproject/amphtml/pull_requests) that happens when you send a pull request are sufficient, but if you want to run your tests across multiple environments/browsers before sending your PR we recommend using Sauce Labs as well.
 
 To run the tests on Sauce Labs:
 
@@ -223,6 +223,10 @@ To run the tests on Sauce Labs:
    ```
 * It may take a few minutes for the tests to start.  You can see the status of your tests on the Sauce Labs [Automated Tests](https://saucelabs.com/beta/dashboard/tests) dashboard.  (You can also see the status of your proxy on the [Tunnels](https://saucelabs.com/beta/tunnels) dashboard.
 
+Cross-browser Testing Platform and Open Source <3 Provided by [Sauce Labs][homepage]
+
+[homepage]: https://saucelabs.com
+
 ## Visual Diff Tests
 
 In addition to building the AMP runtime and running `gulp test`, the automatic test run on Travis includes a set of visual diff tests to make sure a new commit to `master` does not result in unintended changes to how pages are rendered. The tests load a few well-known pages in a browser and compare the results with known good versions of the same pages.