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!