[AIRFLOW-YYYY][PoC] Running tests with pytest

[turbaszek]

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Description

• Here are some details about my PR, including screenshots of any UI changes:
  I create this draft to invite anyone interested in help in creating working proof of concept.
  I tried to keep required changes to minimum but current test suite is so unstable that is hard to keep up with actual master.

What I have already learned is that:

• many tests depend on database
• order of tests is important
• there are tests that create side effects in logging setup, local settings, config.

I am open to any suggestions and help.

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

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Documentation

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