Add unit tests for `morphoclass.features.non_graph`

In #46 we removed various submodules of `morphoclass.feature_extractors` (now called `morphoclass.features`), i.e. `interneurons, pyramidal_cells, feature_extraction`.
Therefore, we also removed the corresponding unit tests for these submodules.

However, we don't have any unit tests for the only submodule left, i.e. `morphoclass.features.non_graph`.

To clarify, it was already a problem before #46, we just noticed the issue there.

_Originally posted by @FrancescoCasalegno in https://github.com/BlueBrain/morphoclass/pull/46#discussion_r870071891_