feat(utxo-lib): add serialized finalized PSBT to test fixtures

Include finalized PSBT in base64 format in test fixtures when available
to help with debugging and verification.

Issue: BTC-2742

Co-authored-by: llm-git <[email protected]>

Author: OttoAllmendinger

modules/utxo-lib/test/bitgo/psbt/SignVerifyPsbtAndTx.ts

@@ -156,6 +156,7 @@ function runPsbt(
       const fixture = {
         walletKeys: rootWalletKeys.triple.map((xpub) => xpub.toBase58()),
         psbtBase64: psbt.toBase64(),
+        psbtBase64Finalized: finalizedPsbt ? finalizedPsbt.toBase64() : null,
         inputs: psbt.txInputs.map((input) => toFixture(input)),
         psbtInputs: getFixturePsbtInputs(psbt, inputs),
         psbtInputsFinalized: finalizedPsbt ? getFixturePsbtInputs(finalizedPsbt, inputs) : null,