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/
issue