chore: bump Jest to v26

[JLHwung]

Q	A
Any Dependency Changes?	Bumped Jest to v26, but test node 6&8 on Jest 24.
License	MIT

This PR bumps Jest to v26. Since v26 drops Node.js 8 support, we downgrade Jest to v24 on Node 6/8 CI.

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[JLHwung]

We can remove babel-jest because jest-config always load babel-jest and we are not using babel-jest explicitly in jest config.

https://github.com/facebook/jest/blob/e84a70c8456d61daa36b5c79a4df7ee752d6d6d6/packages/jest-config/src/normalize.ts#L241

[nicolo-ribaudo]

Is there any advantage in upgrading, since we have to stick to Jest 24 features anyway for compatibility?

[JLHwung]

@nicolo-ribaudo This PR is extracted from #11968. Jest 26 supports collecting coverage from v8: So we don't have to rebuild artifacts with istanbul when running coverage test. This is great for local development -- to see what is the popular path. However I can not get it working nicely with codecov, because we have to collect coverage from **/lib/**, which is not included in our git repo so codecov can not print the coverage report. It works nicely on local build. I will see if I can add yarn gulp test-dev-coverage in the future.

Jest 26 should be faster than v24 because it compiles to Node.js 10. I think we could use some Jest 26 features, -- just have to skip the test on Node.js 6 / 8.

Generally bumping Jest to v26 also ensure that we have tests running on Jest 26, eventually we will use Jest 26 on Babel 8, we can provide feedback earlier if any regression introduced, otherwise we may come across surprises when bumping Jest on Babel 8 in the future.

[nicolo-ribaudo]

This feels like an hack, but given that we are using .mjs files from some tests I think we should still merge this PR.

For example, it will make tests for #12266 much easier.