Switch to using `puppeteer` instead of `WebDriver` for all integration tests

[elliette]

We have started using package:puppeteer to write tests for the new Debug Extension.

We should use package:puppeteer for all tests that require interaction with the Chrome browser, eg the devtools_test. This might improve the set up load of these tests.

[bkonyi]

We should revisit this if we ever get puppeteer running on CI.