Additional test fix

Author: sfc-gh-jkew

modin/tests/pandas/extensions/test_pd_extensions.py

@@ -163,6 +163,12 @@ def python_concat(*args, **kwargs):
             == "pandas_concat_result"
         )
 
+        # With inplace casting we need to reset the original dataframes
+        with config_context(Backend="Pandas"):
+            modin_on_pandas_df = pd.DataFrame({"a": [1, 2, 3]})
+        with config_context(Backend="Python_Test"):
+            modin_on_python_df = pd.DataFrame({"a": [4, 5, 6]})
+
         assert (
             pd.concat([modin_on_python_df, modin_on_pandas_df])
             == "python_concat_result"