Run the integration tests in headless Chrome

[kzar]

Puppeteer recently added headless support for running Chrome
extensions[1], so let's make use of that. Also, let's take care to
properly wait for the extension's background page to load.

1 - puppeteer/puppeteer#8260

Reviewer: @jonathanKingston

CC: @kdzwinel, @ladamski

Steps to test this PR:

Verify that integration tests are still passing both on CI and locally.

Automated tests:

• Unit tests
• Integration tests

Reviewer Checklist:

• Ensure the PR solves the problem
• Review every line of code
• Ensure the PR does no harm by testing the changes thoroughly
• Get help if you're uncomfortable with any of the above!
• Determine if there are any quick wins that improve the implementation

PR Author Checklist:

• Get advice or leverage existing code
• Agree on technical approach with reviewer (if the changes are nuanced)
• Ensure that there is a testing strategy (and documented non-automated tests)
• Ensure there is a documented monitoring strategy (if necessary)
• Consider systems implications

[kzar]

(See also my suggested changes to the Puppeteer docs: puppeteer/puppeteer#8321.)

[jonathanKingston]

🔥 nice!