Allow services to be omitted in usage_scenario

[davidkopp]

Quote from an existing source code comment:

technically the usage_scenario needs no services and can also operate on an empty list This use case is when you have running containers on your host and want to benchmark some code running in them

Actually, it was not possible to use an usage_scenario without services. This PR solves it.

This PR also includes a change to the README in the tests directory to make clear, that some more requirements have to be installed to run the unit tests. I hope that's ok.