Refactor tests to use VCR or similar tools

[pmayd]

At the moment the tests mock the GENESIS API so we don't have to call it during tests.

But we don't have a good strategy how to do this or what to mock. We have some samples from GENESIS but not for all databases or rich variety.

We can use tools like vcr that can record API interactions and reply the data, making further requests offline. This could help us achieving better test coverage and more real test data.

TODOS:

• decide about good test cases for all three databases
• record interaction with vcr (pytest plugin!)
• store files in repository
• let tests use these files in the future