document zbar testing dependency

[missytake]

Just set up a development environment on a new machine and noticed that we didn't document the new testing dependency in #106.

The CI already does this automatically in https://github.com/deltachat/mailadm/blob/master/.github/workflows/ci.yml#L17 and https://github.com/deltachat/mailadm/blob/master/tox.ini#L9 - just for human developers it was not documented yet.

[brabo]

looks good!

[link2xt]

There are two unrelated commits cancelling each other, could be removed before merging, otherwise LGTM

[missytake]

CI passes locally with testrun.org (dc.develcow.de still broken)

(offtopic: take a look at my heart attack xD wanted to rebase on master to run CI locally. Fastest ctrl+c I ever pressed:)

user@blank:~/git/mailadm [env] devenv 10s 255 ± git pull
remote: Enumerating objects: 33, done.
remote: Counting objects: 100% (33/33), done.
remote: Compressing objects: 100% (7/7), done.
remote: Total 25 (delta 18), reused 18 (delta 16), pack-reused 0
Unpacking objects: 100% (25/25), 3.10 KiB | 453.00 KiB/s, done.
From mgithub:deltachat/mailadm
   43d2208..9e12a7d  master     -> origin/master
Already up to date.
user@blank:~/git/mailadm [env] devenv 2s ± git push -f origin master 
^C
user@blank:~/git/mailadm [env] devenv 130 ± git pull origin master 
From mgithub:deltachat/mailadm
 * branch            master     -> FETCH_HEAD
Successfully rebased and updated refs/heads/devenv.