Write initial testing style guide

[joshblack]

As we start to refactor our test suite in #8143, we should begin to author a living testing style guide that lives in /docs/style.md. This testing style guide should cover the following topics and should be added to over time:

• Test structure
  • Where should user-driven (e2e) tests be written
  • Where should automated accessibility tests be written
  • Where should Component API tests be written
• Test recipes for common practices

[joshblack]

Initial draft that was last shared: https://gist.github.com/joshblack/c106ccde36780e0bf3fb7d71bc7bf4e2