Tests: Extract e2e test utils to their own package

[gziolo]

Description

Part of #12313.

I did some final tweaks for the code organization related to e2e tests:

• Extracted e2e test utils to their own package with all utils exposed for general usage as @wordpress/e2e-test-utils;
• Renamed @wordpress/tests-e2e to @wordpress/e2e-tests to align with the newly extracted package name.
• Moved all tests that verify the logic that depends on extensibility and test plugins to their own folder: specs/plugins/.
• Reintroduced a simplified config file for e2e tests to make it easier to use it outside of Gutenberg.

How has this been tested?

npm run test-e2e
or
./node_modules/.bin/wp-scripts test-e2e --config ./packages/e2e-tests/jest.config.js

[gziolo]

@youknowriad - I haven't found a good way to run those tests from withing node_modules subfolder. We probably will have to use symlink with WordPress core to make it work. See related issues:

• Jest not running tests in src/node_modules jestjs/jest#2145
• Jest ignoring tests inside src/node_modules facebook/create-react-app#1042
• Only ignore root /node_modules/ jestjs/jest#1995
  There is much more. In general Jest by default ignores everything in node_modules folder to speed up bootstrap. It makes a lot of sense so I wouldn't fight against it but rather use symlinks or copy tests to a different location.

[gziolo]

@gwwar @Copons - not sure if you are still looking into increasing code reuse from core for your e2e tests setup, but this PR tries to expose all test utils as a npm package. We exposed npm scripts which runs the e2e tests setup in one of the previous PRs. Once this code is published to npm, it should be easier to replicate a similar testing workflow.

[gziolo]

@ryelle and @greatislander - we are close to having all parts of Gutenberg's setup exposed to npm. It may take a few more days (hopefully not weeks) to have it sorted out. It should drastically simplify the way you can use it outside of Gutenberg. I'm going to work on having tooling for a11y included in this setup as my next task.

[jorgefilipecosta]

Currently, we were not able to execute a single end to end test case (e.g.: executing: npm run test-e2e packages/e2e-tests/specs/sidebar.test.js) this PR fixes that. Although I don't have deep knowledge of the changes they make sense to me 👍