Improve test coverage on key API routes

[leftieFriele]

In the service, there are hardly any tests ensuring the behaviour don't change when we perform major upgrades to dependencies. This was exemplified in #592 where a major update in one plugin caused compression to stop working.
The change in the plugin didn't cause errors or anything, it just wasn't registered like it was before.

If there'd been some tests on a core features such as "getting compressed assets", this would've been avoided. We should add tests for similar critical API endpoints in the service to ensure similar mishaps happen later.