Better test cases for user API

In brief, the current test cases don't test anything except that the server is running and does respond to requests. Moreover, the API is inconsistent and incomplete.

I created a couple of test cases on the user API to illustrate a more consistent and complete approach using the user API as an example.
We will end up with a lot of tests, so it makes sense to put them into separate files based on the Resource tested (/users, /measurements, etc.)