Rework the test framework for the package management scripts

[tkilias]

Background

• We currently use some bash scripts to the package management scripts.
• Because of install_via_pip.pl might downgrade already installed packages  #794 the tests get more complicates such that we should move these tests to python.
• We have two types of tests:
  1. Simple unit tests which compare the generated commands
  2. System-tests which run the commands in a docker container
• Currently, both use the same scripts; however, the system-tests can only test of the commands are running without failing, but not the changes of the environment
• Further, currently share all tests of one script, the same docker container

Acceptance Criteria

• Implement the tests as pytest tests
• Use a fixture to create a new docker container for each system-test
• Add checks of the environment for the system-tests

[tkilias]

Blocked, because it needs #242