TST: Mark groupby.nunique test as slow
This test takes about 40 seconds to run for me while shrinking the size of the data brings the runtime down to 2 seconds, presumably without changing coverage (do we even need to parametrize over n and m here?).
I've also wondered why the parameters are so large. Maybe it'd be worth tracking down the original issue and adding a comment.  > I've also wondered why the parameters are so large. Maybe it'd be worth tracking down the original issue and adding a comment.

Looks like it was added here: https://github.com/pandas-dev/pandas/pull/10894 thanks @dsaxton 