Create unit tests and separate them from integrated/e2e tests

[umitkaanusta]

There are many components of reddit-detective which can and should be unit tested. Add unit tests for those stuff, separate them from e2e tests

[umitkaanusta]

Tests should measure cost, speed, and confidence. Unit tests won't be reliable in many cases since most of the functions purposefully directly interact with the DB.

For example, analytical functions can be unit tested by using a graph data structure implemented in Python. However, these tests might not realistically measure speed or confidence. reddit-detective is designed to interact with Neo4j, not with a graph implemented in Python.