cleanup: remove jafar and the previous QA framework (#1244)

## Checklist

- [x] I have read the [contribution
guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request:
ooni/probe#1803
- [x] if you changed anything related to how experiments work and you
need to reflect these changes in the ooni/spec repository, please link
to the related ooni/spec pull request: N/A
- [x] if you changed code inside an experiment, make sure you bump its
version number: N/A

## Description

This is the final act of the quest to replace Jafar. Let's remove Jafar
itself and the QA framework using it. We now have a much better QA
framework, that runs for each commit, is faster, and allows to better
emulate censorship cases.

While working on the removal, I noticed some tutorials depended on a
limited set of Jafar functionality, namely the possibility of provoking
simple censorship conditions. So, I refactored the previous code for
managing iptables on Linux used by Jafar to produce a much slimmer,
fully unit tested tool implementing a subset of the original CLI.

By implementing this change, we make sure that we still have simple ways
for people to learn while reading tutorials.

What remains to be done at this point is to update existing issues,
close done issues, and generally make sure we explain clearly what we
achieved by working on this quest, and what new features are now
available.

.gitignore

@@ -16,7 +16,6 @@
 /citizenlab-test-lists
 /gardener
 /ghpublish.out.txt
-/jafar
 /libooniengine.*
 /measurement.json
 /miniooni
@@ -25,4 +24,5 @@
 /ooniprobe
 /ooporthelper
 /probe-cli.cov
+/tinyjafar
 /tmp-*

CONTRIBUTING.md

@@ -88,10 +88,9 @@ will understand you want to use the default pool)
 ## Code testing requirements
 
 Make sure all tests pass with `go test -race ./...` run from the
-top-level directory of this repository. (Integration tests may be
-flaky, so there may be some failures here and and there; we know
-in particular that `./internal/cmd/jafar` is one of the usual
-suspects and that it's not super pleasant to test it under Linux.)
+top-level directory of this repository. (Running [netem](
+https://github.com/ooni/netem) based tests may not work as intended
+with `-race` with macOS.)
 
 ## Writing a new OONI experiment