test: add test for ndim groupby

[FabianHofmann]

addresses #299

TODO:

• fix fallback implementation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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