Exercise the cache's read path in the equivalence test

Both psbts carried a single change output, so every derivation was a
first-time miss and the cached run only ever wrote to the cache. A
wrong value returned on a hit would have gone unnoticed. Carrying the
change output twice makes the second traversal read back what the
first one cached.

tests/test_psbt_parser.py

@@ -658,9 +658,13 @@ class TestPSBTParserOptimizations:
         each cosigner's account xpub.
         """
         def build_psbt(input_base64: str, change_hex: str) -> PSBT:
-            # A fresh psbt for each parse: the base psbt plus its change output
+            # A fresh psbt for each parse: the base psbt plus its change output, twice.
             psbt = PSBT.parse(a2b_base64(input_base64))
             psbt.outputs.append(create_output(change_hex, 10_000))
+
+            # Add a duplicate output to ensure that the cache yields some hits; the second
+            # output will traverse the same levels the first one just cached.
+            psbt.outputs.append(create_output(change_hex, 10_000))
             return psbt
 
         def assert_cache_makes_no_difference(input_base64: str, change_hex: str):