added documentation for running specific tests

[joaquinvanschoren]

What does this PR implement/fix? Explain your changes.

Adds documentation about running specific unit tests. I found this very useful and didn't know this beforehand.

[mfeurer]

Hey, this is really useful, thanks a lot. Could you please extend this a bit by giving advice how to run all unit tests for a module, and also use a unit test which is already inside develop and which will most likely also stay there (for example something testing datasets/functions.py)?

[mfeurer]

Hey, I think that my previous comment still applies and it would be great to have this additional information before merging this pull request.

[codecov-io]

Codecov Report

Merging #561 into develop will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #561      +/-   ##
===========================================
- Coverage     89.9%   89.87%   -0.04%     
===========================================
  Files           32       32              
  Lines         3031     3031              
===========================================
- Hits          2725     2724       -1     
- Misses         306      307       +1

Impacted Files	Coverage Δ
openml/runs/functions.py	87.3% <0%> (-0.23%)	⬇️

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[mfeurer]

Amazing, thanks a lot!