dillon1000/react

Enable linting for __tests__

Closes #3971.

> After #3968, the next thing we should do is start linting our tests.
> Historically we've ignored them due to lack of parser compatibility.
> But that shouldn't be a problem anymore. We may want to integrate
> https://www.npmjs.com/package/eslint-plugin-react to more aggressively
> lint our JSX in tests.

I understand this diff touches a lot of stuff, so I tried to keep it to
a near-minimal set of changes to make eslint happy.