Add smoke tests

- extend `pytest` with an option to run only the minimum number
  of (unit) test cases to just keep the coverage at 100%
- rationale:
  + many of the unit test cases partly overlap with
    respect to the lines of source code executed
  + also, integration tests, by definition, do not
    contribute to a higher test coverage
- implementation: mark "redundant" test cases as one of:
  + `pytest.mark.integration_test`
    => code usage from the perspective of the end user
  + `pytest.mark.overlapping_test`
    => tests not contributing to the 100% coverage
  + `pytest.mark.sanity_test`
    => tests providing confidence in the test data
- add `tests.conftest` module
  => programatically convert the above markers into
     `@pytest.mark.no_cover` and collect the non-"redundant" tests
- add nox session "test-fast" to run only the minimum
  number of (unit) test while holding coverage at 100%
- refactor some test modules
  + wrap some test cases in a class
  + move sanity tests to the end of the files