v0.4.12: Add unit and integration test environment (#44)

* add test environment
add 1st unit test
add 1st integration test

* add license header

* Delete unnecessarily docu

* revert build.yaml

* add license header to counterfeiter generating

* add license header

* remove duplicate constants

* check metrics port

* remove netstat

* try metrics-port 8088

* disable test in dockerfile

* disable metrics for envtest

* revert removal of envtest in dockerfile

---------

Co-authored-by: TheBigElmo <50952498+TheBigElmo@users.noreply.github.com>
Co-authored-by: d039803 <ralf.hammer@sap.com>