Adding integration tests

What does this MR do and why?

Adding integration tests

Addressing part of gitlab-com/gl-infra/tenant-scale/cells-infrastructure/team#253 (closed)

How to set up and validate locally

Testing a working version

1. cd integration/server; npm ci # to install dependencies for the fake upstream
2. cd ../; npm test # to run the integration tests

Reproducing the bug

1. git revert 8c67965c170cce748e4f25907ac7f2bd4e7354b8
2. Run the integration tests again. They should fail.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Closes #253