add test for grouped column

Author: samukweku

tests/dt/test-nth.py

@@ -144,8 +144,7 @@ def test_nth_grouped():
     assert_equals(DT_nth, DT_ref)
 
 
-def test_nth_grouped_by():
-    """Test nth when called on the `by` column"""
+def test_nth_grouped_column():
     DT = dt.Frame([0, 1, 0])
     DT_nth = DT[:, dt.nth(f.C0, 0), by(f.C0)]
     DT_ref = dt.Frame({'C0':[0,1], 'C1':[0,1]})