WIP: Migrate to jest

[danez]

Q	A
Patch: Bug Fix?	No
Major: Breaking Change?	No
Minor: New Feature?	Yes
Deprecations?	No
Spec Compliancy?	Yes
Tests Added/Pass?	Yes
Fixed Tickets	Fixes #5197, Fixes #5359
License	MIT
Doc PR	No
Dependency Changes	Yes

Rebase and squash of #5359

This does not work correctly so far because jestjs/jest#4393 is not released yet.

Some tests are not working and I haven't found a solution or did not look at them yet.

• All babel-register tests fail, because we cannot really use babel-register inside the jest-node-environment and we do not have access to a real require inside the test. Not sure what we should do here. Even if i try to write our own environment that runs in the same context, require would still be hijacked by jest and not do the real internal require cycle I think?
• babel-plugin-transform-es2015-block-scoping/general › label fails because babylon cannot parse the source.
• packages/babel-core/test/api.js - complex plugin and preset ordering is failing because the order does not match. Not sure why.

//cc @cpojer

[mention-bot]

@danez, thanks for your PR! By analyzing the history of the files in this pull request, we identified @existentialism, @hzoo and @motiz88 to be potential reviewers.

[thymikee]

@danez we recently released new major.

[danez]

I dived deep into the jest source just to find out that I don't see any way we can use jest to test babel-register. Require hooks seem to not be testable with jest, because jest has it's own require implementation that does not respect require hooks.

So we could either stay with mocha, or migrate to ava (meh) or use jest and only for babel-register use another test framework? Thoughts?

[thymikee]

@danez I actually encountered similar issues testing babel-register dependent code, and the workaround was to use Node's module directly, which was pain.
However, now you could use jest-mocha-runner for testing babel-register code (Jest can run Mocha as a test runner). cc @rogeliog

[SimenB]

@thymikee: #6442