feat(utxo-staking): add separate tests for PSBT signature validation

Add separate test cases for validating signatures in the PSBT and for
matching against fixture data in the unbonding transaction tests.
This provides clearer test structure and better isolation of concerns.

Issue: BTC-2319

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

Author: OttoAllmendinger

modules/utxo-staking/test/unit/babylon/undelegation.ts

@@ -5,6 +5,7 @@ import { PathReporter } from 'io-ts/lib/PathReporter';
 import { isLeft } from 'fp-ts/Either';
 import * as utxolib from '@bitgo/utxo-lib';
 import { ast, Descriptor } from '@bitgo/wasm-miniscript';
+import { PartialSig } from 'bip174/src/lib/interfaces';
 
 import { toPartialSig, UndelegationResponse } from '../../../src/babylon/undelegation/UndelegationResponse';
 import { assertValidSignatures, toUnbondingPsbtWithSignatures } from '../../../src/babylon/undelegation/unbonding';
@@ -37,15 +38,18 @@ function toDescriptor(descriptor: DescriptorLike): Descriptor {
 
 function runTest(network: utxolib.Network, txid: string, descriptor: Descriptor): void {
   describe(`Unbonding transaction ${txid}`, function () {
-    it('should create a PSBT from the unbonding transaction', async function () {
+    let psbt: utxolib.bitgo.UtxoPsbt;
+    let signatures: PartialSig[];
+
+    before('should create a PSBT from the unbonding transaction', async function () {
       const fixture = await getFixture(txid);
       const txBuffer = Buffer.from(fixture.unbonding_tx_hex, 'hex');
       const tx = utxolib.bitgo.createTransactionFromBuffer(txBuffer, network, {
         amountType: 'bigint',
       });
-      const signatures = fixture.covenant_unbonding_sig_list.map((sig) => toPartialSig(sig));
+      signatures = fixture.covenant_unbonding_sig_list.map((sig) => toPartialSig(sig));
 
-      const psbt = toUnbondingPsbtWithSignatures(
+      psbt = toUnbondingPsbtWithSignatures(
         tx,
         {
           script: Buffer.from('5120b81e8691ab3c46bddf8ea43f776e4d64abf417a0fa5a4d09976c8aaadabbb6a0', 'hex'),
@@ -55,9 +59,13 @@ function runTest(network: utxolib.Network, txid: string, descriptor: Descriptor)
         signatures,
         utxolib.networks.testnet
       );
+    });
 
+    it('should have valid signatures', async function () {
       assertValidSignatures(psbt, 0, signatures);
+    });
 
+    it('should match the fixture', async function () {
       await assertTransactionEqualsFixture(
         'test/fixtures/babylon/unbonding.' + txid.substring(0, 4) + '.psbt.json',
         psbt