Simplify and improve E2E Puppeteer Config

Bug Description

As pointed out by @aaemnnosttv, @wordpress/scripts already supports passing some custom configs to Puppeteer, so that we don’t actually need to maintain a custom jest-puppeteer.config.js file.

More specifically, it allows setting PUPPETEER_HEADLESS to disable headless mode and PUPPETEER_SLOWMO to slow down tests.

Example:

PUPPETEER_HEADLESS=false npm run test:e2e
PUPPETEER_SLOWMO=200 npm run test:e2e

There’s also a third option, passing a --puppeteer-interactive argument to the command that would result in non-headless mode with a slowMo value of 80. In other words, npm run test:e2e -- --puppeteer-interactive is equivalent to running

PUPPETEER_HEADLESS=false PUPPETEER_SLOWMO=80 npm run test:e2e

Second, the default config also ensures that the --runInBand argument is always passed to Jest, which can speed up tests on CI servers in some cases.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

• More straightforward and better documented way of running E2E tests
• Tests run serially on Travis CI

Implementation Brief

• Remove custom jest-puppeteer.config.js file.
• Alter documentation to suggest using PUPPETEER_HEADLESS / PUPPETEER_SLOWMO or the --puppeteer-interactive argument.
• Add new npm script npm run test:e2e:interactive that runs npm run test:e2e -- --puppeteer-interactive
• Add new npm script npm run test:e2e:ci that runs npm run test:e2e -- --runInBand
• Change .travis.yml to use npm run test:e2e:ci

Changelog entry

• N/A

[circlecube]

That’s right, sorry for the lapse, thanks for providing the link @swissspidy.

Ready for CR and added suggestions to the contributor handbook doc.

[swissspidy]

Current documentation is in the contributor handbook: https://docs.google.com/document/d/1M1gUI0FKnP3RcbgMsU6YltEtNCHaH_9YGbH8F-7JefA/edit#heading=h.21tk21w0qtqj

My understanding is that this doc will be moved to the wiki here on GitHub eventually.