Expose internal testing framework for writing unit tests for steps

[p12tic]

This PR exposes part of our internal test framework and enables the users to write unit tests for steps.

We currently mark the API as experimental because we may still discover improvements of how to expose this.

I'm not completely sure if buildbot.test.expect is the best place for command expectations. Maybe they should be moved to buildbot.test.steps.

cc @tardyp

Contributor Checklist:

• [n/a] I have updated the unit tests
• I have created a file in the newsfragments directory (and read the README.txt in that directory)
• I have updated the appropriate documentation

[codecov]

Codecov Report

Merging #6291 (6b23422) into master (1c72c62) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6291      +/-   ##
==========================================
- Coverage   91.92%   91.91%   -0.02%     
==========================================
  Files         334      334              
  Lines       37542    37542              
==========================================
- Hits        34511    34505       -6     
- Misses       3031     3037       +6     

Impacted Files	Coverage Δ
master/buildbot/util/queue.py	90.69% <0.00%> (-5.82%)	⬇️
worker/buildbot_worker/tunnel.py	88.13% <0.00%> (-1.70%)	⬇️

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