Add detailed assert message to IndexAuditUpgradeIT (#30669)

Print out the returned buckets if the size does not match the
expectation.

Author: tvernum

x-pack/qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/IndexAuditUpgradeIT.java

@@ -123,6 +123,6 @@ private void assertNumUniqueNodeNameBuckets(Matcher<Integer> numBucketsMatcher)
         assertNotNull(nodesAgg);
         List<Map<String, Object>> buckets = (List<Map<String, Object>>) nodesAgg.get("buckets");
         assertNotNull(buckets);
-        assertThat(buckets.size(), numBucketsMatcher);
+        assertThat("Found node buckets " + buckets, buckets.size(), numBucketsMatcher);
     }
 }