Persistency ouput test

[Durman]

Addressed problem description

Closes #4226

1. Get json from the sverchok/json_examples folder if there is json with the same name
   in sverchok/tests/expected_tree_data folder
2. Import json into a new tree
3. Execute the tree
4. Extract values from most right nodes in the tree
5. Compare them with values from the expected_tree_data folder and raise an error if necessary
6. Save coverage information into expected_tree_data/last_coverage.json

The test uses 45 json examples now. It covers 151 nodes what is about 22% of all nodes. But these nodes are not tested even in half of their possible modes. So total coverage is estimated about 9%. It means that there is work ahead in this direction. It would be cool if examples in the documentation to each node could be imported too. In this case they could be used in the test and coverage would be much better.

Preflight checklist

• Code changes complete.
• Code documentation complete.
• Documentation for users complete.
• Ready for merge.