Make React tests fast

[gaearon]

Currently we have a single giant file with all tests, and a giant snapshot. This is both slow, and hard to work with and iterate on.

In this PR I will refactor our test setup.

• Split it up into multiple files (gets the test running from 45s to 27s)
• Run Prettier on test files
• Split tests further for better performance
• Make it possible to run one test file
• Fix the issue with double test re-runs in watch mode on changes in the test file
• Refactor error handling
• Run Prettier on fixtures
• Add a fast mode with yarn test-react-fast <Filename>
• Fix double reruns on failure

Potential followups:

• Figure out why test interruption broke (need Run single-file test in a process if we know it takes long jestjs/jest#6599 and "Enter" doesn't interrupt tests anymore jestjs/jest#6598 fixed)
• Revisit weird things like this['React'] assignment with a funny comment in every test

[gaearon]

I think this is ready for a review now. I'll check remaining followups either later in this PR, or in the next one.

This reduces the time it takes to run yarn test-react from ~45s on my machine to ~21s. It also fixes an annoying issue where a watcher would trigger twice on each change, and clear the screen while you were trying to read it.

It gives you an ability to run a subset of tests from command line, e.g. yarn test-react FirstRender. You can do the same with Jest watcher filtering interface in yarn test-react --watch (and then press p).

When there's a bug in one output mode (e.g. createElement => createElement), it now fails immediately for a particular test case, and doesn't attempt it in other modes (since they would likely fail too). So failures should appear faster.

Additionally, this PR adds yarn test-react-fast for cases when you don't care about JSX syntax tests. It runs all tests in ~10s (and is even faster in watch mode), but the output is a bit noisy because of a warning about "obsolete snapshots" (which aren't really obsolete—just skipped in this mode).

I also added Prettier to tests so we can stop doing the formatting nits.

[trueadm]

Great work!

[gaearon]

Okay, I figured out the last pieces.
Jest tests don't pause properly because of jestjs/jest#6598 and jestjs/jest#6599.
Once that gets fixed, we'll get a fast and responsive test runner.

[facebook-github-bot]

@gaearon is landing this pull request. If you are a Facebook employee, you can view this diff on Phabricator.