Improve unit tests using golang

In #14 we added some basic unit tests. The goal of this issue is to improve these tests. It would be nice if we were able to make our testing completely independent of kubernetes. So that we don't have to rely on the provision of clusters to be sure we don't break functionality with new changes.

Possibly good starting point: https://github.com/operator-framework/operator-sdk/blob/master/doc/user/unit-testing.md