Fix nox task test:coverage #249

BFSPY has a local implementation for nox task test:coverage:

• The docstring for function coverage() is “Runs all tests (unit + integration) and reports the code coverage"
• While the implementation of _coverage() calls _unit_tests(session, config, context)

Questions

• Is this (local) Nox task still required wrt. the PTB's default implementation?
• Should the PTB be updated?
• Should code coverage only be evaluated by Sonar?