fix(z:acceptance): fix flakey z:acceptance/vaults.test.js (#10643)

closes: #10599

## Description

#10599 pointed out a flakiness in `z:acceptance/vaults.test.js`. Investigations showed the problem was because of the batched IST fee which is charged for every 10 offers for a given user. We started using `tryISTBalance` instead of a direct assertion. 

### Security Considerations

None.

### Scaling Considerations

Making sure there are no flakiness in `a3p-integration/z:acceptance` tests is important as it  affects other PRs in the CI. This should fix one of the discovered ones.

### Documentation Considerations

None.

### Testing Considerations

This should unblock other PRs CI process. No other considerations.

### Upgrade Considerations

`z:acceptance` tests run before every upgrade.

a3p-integration/proposals/z:acceptance/vaults.test.js

@@ -226,8 +226,7 @@ test.serial('user cannot open a vault above debt limit', async t => {
   );
 });
 
-// TODO #10599. marked as `skip` since several flakes were observed
-test.skip('user can open a vault under debt limit', async t => {
+test.serial('user can open a vault under debt limit', async t => {
   const istBalanceBefore = await getISTBalance(GOV1ADDR);
   const activeVaultsBefore = await listVaults(GOV1ADDR, walletUtils);
 
@@ -241,11 +240,12 @@ test.skip('user can open a vault under debt limit', async t => {
   const istBalanceAfter = await getISTBalance(GOV1ADDR);
   const activeVaultsAfter = await listVaults(GOV1ADDR, walletUtils);
 
-  t.is(
-    istBalanceBefore + Number(mint),
-    istBalanceAfter,
-    'The IST balance should increase by the minted amount',
+  await tryISTBalances(
+    t,
+    scale6(istBalanceAfter),
+    scale6(istBalanceBefore + Number(mint)),
   );
+
   t.is(
     activeVaultsAfter.length,
     activeVaultsBefore.length + 1,