Graph Generation: Ensure that optional tests are executed and not skipped

[meretp]

#562 added an optional feature to generate a graph representing all relationships defined in the document. To run this feature the user needs to install the package networkx. As this is a rather big dependency we decided to make it optional. According to that the tests for the new feature are skipped if this package is not installed. The GitHub Action to run all tests manually installs this dependency and runs the tests in a second stepo to ensure that all tests pass (see #568). In this PR the question came up if we can check that the tests are actually run and not skipped. Possible solutions could be either to check the generated output from pytest or define an environment variable to disable skipping of tests.