Move/add instrumentation tests to automation/Extension/webext-instrumentation

[motin]

These tests should be decoupled from the framework tests.

[motin]

To ensure that the tests are decoupled from the framework code, it not only needs its own tests, but also its own way of executing those tests.

Unit tests are already set up, but any test that relies on web extension APIs requires testing using a real browser.

I dug around to find the most up to date best practices for testing web extensions using a real browser, and it seems that proper tooling that supports Web Extension developer workflows is still rare, and such tooling that supports Web Extension that uses Web Extension Experiments outside the Firefox source code tree is even rarer.

Findings summarized in motin/firefox-webext-test-automation-example#1

TLDR: Taken all of this into account, my intention is to make https://github.com/mozilla/shield-studies-addon-utils/tree/master/testUtils available outside of the study utils package and then set up Selenium-based tests for webext-instrumentation. I'll possibly evaluate puppeteer-firefox and try out theintern.io as well in order to see if the current best practices can be improved upon.

[vringar]

I don't think we'll ever separate the instrumentation from OpenWPM ever again.