Fix test__get_dtype test expecting little-endian (#20662)

Similar to #14832, use = (native) instead of < (little-endian)

Author: ginggs

pandas/tests/dtypes/test_common.py

@@ -570,8 +570,8 @@ def test_is_offsetlike():
     (pd.CategoricalIndex(['a', 'b']), CategoricalDtype(['a', 'b'])),
     (CategoricalDtype(), CategoricalDtype()),
     (CategoricalDtype(['a', 'b']), CategoricalDtype()),
-    (pd.DatetimeIndex([1, 2]), np.dtype('<M8[ns]')),
-    (pd.DatetimeIndex([1, 2]).dtype, np.dtype('<M8[ns]')),
+    (pd.DatetimeIndex([1, 2]), np.dtype('=M8[ns]')),
+    (pd.DatetimeIndex([1, 2]).dtype, np.dtype('=M8[ns]')),
     ('<M8[ns]', np.dtype('<M8[ns]')),
     ('datetime64[ns, Europe/London]', DatetimeTZDtype('ns', 'Europe/London')),
     (pd.SparseSeries([1, 2], dtype='int32'), np.dtype('int32')),