[fix] flaky test in proposer (MystenLabs#9809)

## Description 

This PR is fixing a flaky test in Narwhal and more specifically when
testing our gRPC Proposer API . Basically the order of the
AuthorityFixtures wasn't aligned with the produced Committee
authorities. That made the `test_rounds_errors` test fail randomly as it
is based on parsing both the Committee and AuthorityFixtures.

Also refactored the test to iterate over the authorities using a common
approach via the `fixture.authorities()`

## Test Plan 

How did you test the new or updated feature?

---
If your changes are not user-facing and not a breaking change, you can
skip the following section. Otherwise, please indicate what changed, and
then add to the Release Notes section as highlighted during the release
process.

### Type of Change (Check all that apply)

- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

narwhal/primary/tests/integration_tests_proposer_api.rs

@@ -90,7 +90,7 @@ async fn test_rounds_errors() {
     // In this way, the genesis certificate is not run for that authority and is absent when we try to fetch it
     let mut builder = CommitteeBuilder::new(Epoch::default());
 
-    for authority in committee.authorities() {
+    for authority in fixture.authorities().map(|a| a.authority()) {
         if authority.id() != authority_id {
             builder = builder.add_authority(
                 authority.protocol_key().clone(),

narwhal/test-utils/src/lib.rs

@@ -749,6 +749,10 @@ impl<R: rand::RngCore + rand::CryptoRng> Builder<R> {
             authority.authority = OnceCell::with_value(a.clone());
         }
 
+        // now order the AuthorityFixtures by the authority id so when we iterate either via the
+        // committee.authorities() or via the fixture.authorities() we'll get the same order.
+        authorities.sort_by_key(|a1| a1.authority().id());
+
         CommitteeFixture {
             authorities,
             committee,