Group common jest config in to a preset

[scinos]

Creates a jest preset to have the config for all packages in a common place.

After reading Jest docs and source code, there are a few points that have influenced this decision:

• Files referenced by project doesn't imply any type of extension. The config on the "parent" jest.config.js is not propagated to files referenced by project. Reading config and projects is handled in https://github.com/facebook/jest/blob/master/packages/jest-config/src/index.ts#L273

• The only mechanism provided by jest to do extensions are presets. However it is only one-level: a preset can't have presets. Eg: https://github.com/facebook/jest/blob/master/packages/jest-config/src/normalize.ts#L519 is called when normalizing a config file, but setupPreset is not recursive.

• By default, rootDir is __dirname, no need to specify it again (https://github.com/facebook/jest/blob/master/packages/jest-config/src/readConfigFileAndSetRootDir.ts#L96)

Testing instructions

• Verify tests still pass

[matticbot]

Test live: https://calypso.live/?branch=fix/jest-preset-for-packages

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[sirbrillig]

I don't fully understand the implications, but I don't have any objection WRT the composite-checkout package. The tests still pass, so I'm happy. Do you also need to convert the jest.config.js in packages/shopping-cart?

[jsnajdr]

Do you also need to convert the jest.config.js in packages/shopping-cart?

Yes, the shopping-cart package should also use the new preset. I pushed the change in 704d376

[jsnajdr]

The new common config works for me very well! 👍

By default, rootDir is __dirname, no need to specify it again

I tried to get rid of rootDir some time ago, but failed. It was because I was trying to extend a config from another module:

const commonPreset = require( '../common/jest.config.js' );
module.exports = {
  ...commonPreset,
  foo
}

But that didn't work and revealed a Jest bug. Extending the preset is much better!

[scinos]

Note: ICFY shows a reduction in bundle size but it is not real. It assumes this branch is targeting master, but it is not.

[jsnajdr]

It assumes this branch is targeting master, but it is not.

That's true, ICFY always looks for a branching point from master and calculates the diff from there: https://github.com/Automattic/wp-calypso/blob/master/.github/workflows/icfy-stats.yml#L45