Add unit tests for data generation code

[pdmnyberg]

The data generation code needs tests in order to achieve the following:

• Maintain a robust data pipeline
• Avoid introducing bugs
• Make sure the generated data fullfills the requirements of the expected queries

Acceptance critera

• There are unit tests
• The tests cover important parts of the data generation code
• The tests verify that the data fullfills the requirements of the expected neo4j queries