licensedcode `detection.py` lacks unit tests

We have a lot of full tests checking individual cases of detection issues here, but we likely miss many cases as these are not proper unit tests. See for example the case of bugs in `is_false_positive` function at https://github.com/aboutcode-org/scancode-toolkit/pull/4009 where this was fixed.

We need to ensure all the cases are accounted for and tested with unit tests.