* feat: add end-to-end testing to this repository
Part of https://github.com/ooni/probe-engine/issues/1181
Motivation: we want to run this check from the repository where
we work the most, such that it's unlikely it pauses due to inactivity,
as it may happen for less frequently touched upon repositories.
Code adapted from https://github.com/ooni/e2etesting/
* fix: correct name for main branch
