Document available test data

[VladimirShitov]

Every time writing tests, I wonder if I should create a script for downloading new test data or if I can reuse the existing datasets. Sadly, existing test resources are hardly documented (only with comments in the code of resources_test_scripts). I suggest adding documentation for existing data with the following structure:

• Name or path to the dataset
• Existing fields (output of print(mdata) should be enough