Add tests and test utils for testing persistent and restart features

[motin]

Necessary to verify correct behavior for restart, startup, shutdown and similar events.
Test utils / tests should be available also for use in study add-ons making it easy to test these events in the study add-on directly as well.

[motin]

Official web-ext docs are https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Testing_persistent_and_restart_features, which is handy for manual testing, but the automated selenium/webdriver/geckodriver tests needs to be extended primarily for this issue to be considered fixed.