Add tests to check the API works as expected

[sillitoe]

We could do with lots more tests everywhere, however most importantly we want to confirm that this client has implemented the 3D Beacons API correctly.

An overview of this testing strategy:

• build an API client from the OpenAPI spec (ie independent of any code in this repo)
• get example input data
• start up the test server
• populate the local database with this example data
• check the actual results against the results we expected
• clean up the test files / database

To be honest, this would be the general strategy to test any 3D-Beacons client, so it might even be worth trying to make it as generic as possible,