Add support for testing the WebExtensions API in WPT

rfcs/web_extensions.md

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+# RFC #219: Add support for WebExtensions in WPT
+
+## Summary
+
+The WebExtensions API extends the capabilities of the browser. Adding support
+for WebExtensions in the web platform tests will increase interoperability
+and will help drive the standardization of this API.
+
+This RFC proposes adding a new testharness test type `.extension.js` to handle|
+testing this API, in addition to using `test_driver` to load and unload extensions.
+
+##  Details
+
+Using test_driver, we add support for testing the WebExtensions API by loading
+a web extension designed to test the functionality of a specific API.
+The extension will be loaded after the tests begins, and unloaded before the
+test is finished.
+
+Most of the test execution is handled within the extension, via the 
+[browser.test](https://chromium.googlesource.com/chromium/src.git/+/master/extensions/docs/testing_api.md)
+API. We’ve elected to use these APIs since all participating browser vendors use
+`browser.test` to test the web extensions internally and they can easily port over
+existing tests to the web platform tests.
+
+Because these tests won’t leverage testharness.js directly, we’ve introduced a new
+testharness test type `.extension.js`  that will create the necessary boilerplate to
+convert the `browser.test` assertions into the corresponding assertions in the test
+harness.
+
+A proposed patch is available at [insert WPT link here]
+
+## Alternatives considered
+
+We considered loading the extension statically before each testharness test is run,
+but we decided against that since test_driver will allow the tests to drive the
+browser, and in the future, test functionality such as opening popups and clicking
+menu items. 
+
+We considered adding a new test type other than using JavaScript tests, but we decided
+against it because it was a lot more work compared to using testharness.js. 
+
+
+## Risks
+
+There are two potential concerns with this implementation:
+
+1. We have no precedent for tests run via a Classic command in some user agents
+    and BiDi command in others. However, the Classic implementation of loading
+    and unloading extension is modeled on the BiDi implementation, so we expect the
+    behavior to be the same. The Classic implementation is defined in the WebExtensions Community Group
+    [here](https://github.com/w3c/webextensions/blob/main/specification/webdriver-classic.bs),
+    and the BiDi implementation is defined
+    [here](https://www.w3.org/TR/webdriver-bidi/#module-webExtension).
+
+2. Another concern could be with using `browser.test` assertions and mapping them to
+    `testharness.js` assertions. With `testharness.js` not in charge of generating assertions,
+    we might end up with less useful failure messages.