Make tests more fine-grained (towards TDD)

[CSchoel]

To get closer to a test driven design (TDD) workflow, we need not only test coverage but also fine-grained tests.
Ideally, almost every method in the code should be tested independently.

Among other things, this may require writing dummy server and client objects, in order to decouple method behavior from the actual network connection.

Note: This issue is still work in progress, as it is not designated for the current sprint.