[docs] RTL best practices and naming/structure conventions

[brian-smith-tcril]

During the review process for #2611, a few comments were left about undocumented best practices.

RTL Best Practices

• Use userEvent instead of fireEvent whenever possible (comment)
  • generally preferable since it simulates more of the true user interactions in the browser
  • https://blog.mimacom.com/react-testing-library-fireevent-vs-userevent/
• Avoid relying on class names in tests (comment)
  • if we don't rely on class names tests won't break if/when they change

Naming conventions

• Use kebab-case for test ids (comment)

Structure

• Put common test imports in setupTests.js (comment)
  • DRY is good

It'd be nice to have these (and any other good patterns) documented somewhere (ideally with links to good real-world examples from throughout the codebase).

I'm not sure where the best place for this documentation to live would be. It doesn't quite feel like it fits with the rest of what is in the decisions directory, and it's not exactly Paragon specific.

[brian-smith-tcril]

In order to consolidate discussion around best practices in a non-paragon-specific location, I've created openedx/docs.openedx.org#380 and I will be closing this issue.