Relax pyflakes requirements for test docstrings
Thins to try: * [x] drop requirements that test method docstrings are written in the imperative mood * [x] drop requirements that test method docstrings are present * [x] update coding standards allowing to drop test method docstrings if they a copy of the test method name is sufficient (with examples) * [x] link to https://jml.io/test-docstrings/
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