TST: Mark groupby.nunique test as slow (#34096)

Author: dsaxton

pandas/tests/groupby/test_nunique.py

@@ -9,6 +9,7 @@
 import pandas._testing as tm
 
 
+@pytest.mark.slow
 @pytest.mark.parametrize("n", 10 ** np.arange(2, 6))
 @pytest.mark.parametrize("m", [10, 100, 1000])
 @pytest.mark.parametrize("sort", [False, True])