testjafar.bash

* experiment/stunreachability: skip test consistently failing in GHA

See https://github.com/ooni/probe-engine/issues/874#issuecomment-679850652

* github issue template: customise to ooni/probe-engine

* .github/workflows: simplify scripts

* atomicx: revamp documentation

* CLI: add basic documentation

* cmd: improve the documentation

* testjafar.bash: improve the documentation

* experiment: improve documentation and consistency

* experiment/handler: make internal

This is clearly an internal detail, which should not be public.

* .gitignore: add /cmd/jafar/badproxy.pem

* fix(cmd/jafar): make iptables tests idempotent