itest: add assertion for PSBT change outputs

With this commit we add more specific assertions to our PSBT signing
test in order to make sure change outputs have the proper PSBT metadata
associated with them, depending on their address type.

itest/lnd_psbt_test.go

@@ -1087,6 +1087,34 @@ func assertPsbtFundSignSpend(ht *lntest.HarnessTest, alice *node.HarnessNode,
 	)
 	require.GreaterOrEqual(ht, fundResp.ChangeOutputIndex, int32(-1))
 
+	// Make sure our change output has all the meta information required for
+	// signing.
+	fundedPacket, err := psbt.NewFromRawBytes(
+		bytes.NewReader(fundResp.FundedPsbt), false,
+	)
+	require.NoError(ht, err)
+
+	pOut := fundedPacket.Outputs[fundResp.ChangeOutputIndex]
+	require.NotEmpty(ht, pOut.Bip32Derivation)
+	derivation := pOut.Bip32Derivation[0]
+	_, err = btcec.ParsePubKey(derivation.PubKey)
+	require.NoError(ht, err)
+	require.Len(ht, derivation.Bip32Path, 5)
+
+	// Ensure we get the change output properly decorated with all the new
+	// Taproot related fields, if it is a Taproot output.
+	if changeType == walletrpc.ChangeAddressType_CHANGE_ADDRESS_TYPE_P2TR {
+		require.NotEmpty(ht, pOut.TaprootBip32Derivation)
+		require.NotEmpty(ht, pOut.TaprootInternalKey)
+
+		trDerivation := pOut.TaprootBip32Derivation[0]
+		require.Equal(
+			ht, trDerivation.XOnlyPubKey, pOut.TaprootInternalKey,
+		)
+		_, err := schnorr.ParsePubKey(pOut.TaprootInternalKey)
+		require.NoError(ht, err)
+	}
+
 	var signedPsbt []byte
 	if useFinalize {
 		finalizeResp := alice.RPC.FinalizePsbt(