chore(docs): Add E2E testing ADR

docs/adr/0021-use-internjs-testing.md

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+# Use existing InternJS for functional testing in settings v2
+
+- Deciders: Vijay Budhram, Jody Heavener, Lauren Zugai
+- Date: 2020-07-17
+
+## Context and Problem Statement
+
+For over the past 5 years (at least), we have been using [InternJS](https://github.com/theintern/intern) for our functional tests in the content-server. While this has worked well and caught several bugs, we should reevaluate it since we are migrating to react and have an opportunityto use newer/faster/better supported testing frameworks.
+
+## Decision Drivers
+
+- Ease of writing tests
+- Good community support
+- Learning curve of settings V2 development
+
+## Decision Outcome
+
+Use InternJS for functional testing in the settings v2 app, but also update the tests to support async/await. The biggest factor was not having to introduce another testing framework for developers to learn.
+
+## Considered Options
+
+- Option A - Use InternJS and update to async/await
+- Option B - Use [NightmareJS]()
+- Option C - Use [WebDriverIO](https://webdriver.io/)
+
+## Pros and Cons of the Options
+
+### Option A - Use InternJS and update to async/await
+
+Pros:
+
+- Active community
+- Can reuse the testing patterns from content-server
+- Lots of examples and helper functions to help write tests
+- Developers won't have to context switch with testing frameworks
+
+Cons:
+
+- Still requires a learning curve to understand how things work and are structured
+- It's kind of clunky, would be a lot of effort to go refactor all tests to async/await and could be better to just rewrite them with another library
+- It's an older framework, a newer framework may offer greater support and features
+- Our tests are a little flaky, can another framework help mitigate false failures from CI?
+
+### Option B - Use NightmareJS
+
+Pros:
+
+- Claims to be 2x faster than PhantomJS
+
+Cons:
+
+- Developers/QA will have to learn a new testing framework on top of all the other new frameworks introduced
+- Is chromium based
+
+### Option C - Use WebDriverIO
+
+Pros:
+
+- Active community
+- Used by other Mozilla services
+- Simple API
+- Solid documentation
+
+Cons:
+
+- Developers/QA will have to learn a new testing framework on top of all the other new frameworks introduced