TST: Convert column b in expected to object for pyarrow < 0.15

Closes #27955

Author: galuhsahid

pandas/tests/io/test_parquet.py

@@ -464,7 +464,7 @@ def test_categorical(self, pa):
             check_round_trip(df, pa)
         else:
             # de-serialized as object for pyarrow < 0.15
-            expected = df.assign(a=df.a.astype(object))
+            expected = df.assign(a=df.a.astype(object), b=df.b.astype(object))
             check_round_trip(df, pa, expected=expected)
 
     def test_s3_roundtrip(self, df_compat, s3_resource, pa):