Document test execution and parametrization

[tjkuson]

Document that pytest-asyncio runs test cases sequentially and that it works with pytest.mark.parametrize

Closes #694

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.88%. Comparing base (2c75c45) to head (30500e0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1179   +/-   ##
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  Coverage   88.88%   88.88%           
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  Files           2        2           
  Lines         405      405           
  Branches       44       44           
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  Hits          360      360           
  Misses         35       35           
  Partials       10       10           

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[seifertm]

The "concurrency" topic does come up regularly, so it's nice to have a documentation section to point to.

Thanks!