Create database mocks for better integration-testing

[aistein]

As the project grows in size and scope and more collaborators join, the current methods for testing queries against an actual running instance of Postgres will become cumbersome.

We should implement some common mock-functions for the database that can be used to test all API-calls that interact with the database. These shared mock-database functions could be based around the DataDog go-sqlmock library.

An alternative to this would be to use Test Containers
- a framework in which docker containers running dummy instances of the external dependency (e.g. Postgres) are spun up for each integration-test/ test-group.