TST/DOC: added issue number to test case

Author: winklerand

pandas/tests/test_resample.py

@@ -2854,6 +2854,7 @@ def test_resample_with_nat(self, periods, values, freq, expected_values):
         assert_frame_equal(result, expected)
 
     def test_resample_with_only_nat(self):
+        # GH 13224
         pi = PeriodIndex([pd.NaT] * 3, freq='S')
         frame = DataFrame([2, 3, 5], index=pi)
         expected_index = PeriodIndex(data=[], freq=pi.freq)