Jest: client: set testEnvironment to jsdom

[psealock]

In order to test mounting of components that make use of the window global, this PR changes the jest testEnvironment to jsdom.

Example component:

componentDidMount() {
    window.addEventListener( 'resize', this.handleResize );
}

[matticbot]

Test live: https://calypso.live/?branch=fix/client-jest-testEnv

[psealock]

hmm... 31 failed tests, going to have to look into this one

[gziolo]

You can update individual test suite to use jsdom as described here: https://github.com/Automattic/wp-calypso/blob/master/docs/testing/unit-tests.md#testing-globals.

If you want to update it for all tests, better double check if it won't slow down everything significantly. @nb asked me to check if we could use jsdom by default for all tests, but it slowed down everything by 30%. See related discussion: #16119 (comment).

[gziolo]

BTW, in https://github.com/WordPress/gutenberg we have jsdom set as default, but we have 100+ tests suites, not 1 000+ :D
It would be nice to revisit it and double check if we could switch to jsdom 👍

[psealock]

Closing. Thanks for the links @gziolo