test: add test for ndim groupby

[FabianHofmann]

addresses #299

TODO:

• fix fallback implementation

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.83%. Comparing base (d3703fc) to head (62158fc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #301 +/- ## ======================================= Coverage 89.83% 89.83% ======================================= Files 16 16 Lines 4032 4032 Branches 941 941 ======================================= Hits 3622 3622 Misses 279 279 Partials 131 131 ```

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