Improve Avro unit tests

[bartkl]

Issues:

• When testing message serialization/deserialization, I haven't found a way to let Lancaster deserialize JSON messages, so currently the testdata is simply in EDN form so I can compare immediately. Maybe if I read my JSON file in binary mode as bytes array I can get it to work with l/deserialize, that's worth looking into. Anyways, the thing that needs leveraging is the mapping of JSON field names to the EDN keywords, like `"fromAtoC" -> :from-ato-c'.
• In some test cases it's not necessary to write to file, and simply using the Lancaster schema object immediately makes more sense.
  • Note: core.clj is getting cluttered a bit with high-level user functions that might need to go in some separate namespace at some point (like store-in-db and generate-avro-schema).
• More actual unit tests need to be written anyways...
• Lots of repetition in test logic: use abstractions and fixtures would clean up a lot.
  • In fact, currently when an assertion fails the file-deletion clean-up doesn't even occur, which is technically a bug in the tests.

[bartkl]

Useful comments from previous issue:

Note: make sure to use more meaningful test data than the current single letter data. Having stuff like Car and Vehicle can really make your test data easier to reason about. Now it's totally unclear what relations A and B have to each other until I check.

Probably nice to start testing something stateful like the Asami db. This way we'll learn how to do resource management (fixtures, setup/teardown mechanisms, ...).