TST: Fix date/datetime format test (GH44284)

Swap rows and columns so that columns are homogenous and dates aren't converted to datetimes (i.e. datetime64[ns]) on initialisation of df. Clarify scope of test.

Author: TimoMorris

pandas/tests/io/excel/test_writers.py

@@ -652,25 +652,27 @@ def test_excel_roundtrip_datetime(self, merge_cells, tsframe, path):
 
         tm.assert_frame_equal(tsframe, recons)
 
-    def test_excel_date_datetime_format(self, ext, path):
+    def test_excel_roundtrip_date_datetime_format(self, ext, path):
         # see gh-4133
         #
-        # Excel output format strings
+        # Test that df written with custom date/datetime format strings
+        # is read back the same as if written using the default formats,
+        # and that the values are still recognised as dates/datetimes.
         df = DataFrame(
             [
-                [date(2014, 1, 31), date(1999, 9, 24)],
-                [datetime(1998, 5, 26, 23, 33, 4), datetime(2014, 2, 28, 13, 5, 13)],
+                [date(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+                [date(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
             ],
-            index=["DATE", "DATETIME"],
-            columns=["X", "Y"],
+            index=["X", "Y"],
+            columns=["DATE", "DATETIME"],
         )
         df_expected = DataFrame(
             [
-                [datetime(2014, 1, 31), datetime(1999, 9, 24)],
-                [datetime(1998, 5, 26, 23, 33, 4), datetime(2014, 2, 28, 13, 5, 13)],
+                [datetime(2014, 1, 31), datetime(1998, 5, 26, 23, 33, 4)],
+                [datetime(1999, 9, 24), datetime(2014, 2, 28, 13, 5, 13)],
             ],
-            index=["DATE", "DATETIME"],
-            columns=["X", "Y"],
+            index=["X", "Y"],
+            columns=["DATE", "DATETIME"],
         )
 
         with tm.ensure_clean(ext) as filename2: