Fix flaky RemoteRoutingTableServiceTests.testGetAsyncIndexRoutingTableDiffReadAction (#20618)

The test was failing intermittently with CorruptStateException due to
the mock returning the same stateful InputStream instance in a loop in a
test case. Changed the mock from thenReturn() to thenAnswer() to
generate a fresh input stream for each invocation.

Similarly, a CountdownLatch was being reused within the loop, but would
have been blocking only on the first iteration. Moved the latch to be
initialized inside each loop iteration.

Signed-off-by: Andrew Ross <andrross@amazon.com>

Author: andrross

server/src/test/java/org/opensearch/cluster/routing/remote/RemoteRoutingTableServiceTests.java

@@ -611,14 +611,14 @@ public void testGetAsyncIndexRoutingTableDiffReadAction() throws Exception {
             compressor,
             Version.CURRENT
         );
-        when(blobContainer.readBlob(indexName)).thenReturn(
-            remoteRoutingTableDiff.remoteRoutingTableDiffFormat.serialize(diff, uploadedFileName, compressor).streamInput()
+        when(blobContainer.readBlob(indexName)).thenAnswer(
+            invocation -> remoteRoutingTableDiff.remoteRoutingTableDiffFormat.serialize(diff, uploadedFileName, compressor).streamInput()
         );
 
-        TestCapturingListener<Diff<RoutingTable>> listener = new TestCapturingListener<>();
-        CountDownLatch latch = new CountDownLatch(1);
-
         for (Version version : List.of(Version.CURRENT, Version.V_3_1_0, Version.V_3_2_0)) {
+            TestCapturingListener<Diff<RoutingTable>> listener = new TestCapturingListener<>();
+            CountDownLatch latch = new CountDownLatch(1);
+
             remoteRoutingTableService.getAsyncIndexRoutingTableDiffReadAction(
                 "cluster-uuid",
                 uploadedFileName,