Improve organization of test code

[tuxalp]

• Find an agreement, how test code can be organized in a cleaner way.
• Reorganize test code if necessary

Some test packages are still used in main application code, e.g.:

• test_network_flow

See also:

• Modularize main application code #99

Other suggestions:

• Introduce a module test for all the test packages and make sure that this module is not imported in main app code
• Add unit tests to existing packages and make them callable:

if __name__ == "__main__":
    unittest.main()