Fix flaky BackwardSyncContextTest on slow CI runners (#10000)

shouldSyncUntilHash and shouldSyncUntilRemoteBranch used isCompleted()
inside untilAsserted, which only passes for normal completion. On slow CI
machines the backward sync occasionally completes the future exceptionally,
causing isCompleted() to fail on every retry until the 30s Awaitility
timeout, which then rethrows the AssertionError.

Change to isDone() inside the Awaitility block (matching the fix applied
to shouldAddExpectedBlock in #9856) so Awaitility exits as soon as the
future reaches any terminal state. The subsequent future.get() will then
surface the real exception if the sync failed.

Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: macfarla

ethereum/eth/src/test/java/org/hyperledger/besu/ethereum/eth/sync/backwardsync/BackwardSyncContextTest.java

@@ -303,7 +303,7 @@ public void shouldSyncUntilHash() throws Exception {
         .untilAsserted(
             () -> {
               respondUntilFutureIsDone(future);
-              assertThat(future).isCompleted();
+              assertThat(future).isDone();
             });
 
     future.get();
@@ -337,10 +337,10 @@ public void shouldSyncUntilRemoteBranch() throws Exception {
         .untilAsserted(
             () -> {
               respondUntilFutureIsDone(future);
-              assertThat(future).isCompleted();
+              assertThat(future).isDone();
             });
 
-    future.get(); // Should succeed since we waited for completion
+    future.get();
     assertThat(localBlockchain.getChainHeadBlock()).isEqualTo(remoteBlockchain.getChainHeadBlock());
   }