Bugfixes and more tests

I found it surprising and suspicious that our coverage went from +90% to almost 50%.
While investigating this further, I found some critical bugs.

Moreover, I've added some test sensor values that are loaded into the test database when the technician hooks are tested. This should bump the test coverage to almost 70%.

We still need to write more tests, especially for all hooks that are `POST` only, which aren't tested automatically.