Add more tests to the oonidata related endpoints in oonimeasurements service

[hellais]

The oonidata components could benefit from having better testing: https://github.com/ooni/backend/tree/master/ooniapi/services/oonimeasurements/src/oonimeasurements/routers/data.

This would also give us more confidence in the deployment of the service working as intended: ooni/devops#139.

As part of this it would probably be helpful to think about how to go about generating or provisioning test fixtures. One idea could be for the integration tests to make use of the actual oonipipeline to populate the tables inside of the oonidata service.