Test: Make update-by-query test more resilient

`UpdateByQueryWhileModifyingTests#testUpdateWhileReindexing`
runs update-by-query and concurrently updates, asserting that
the update-by-query never reverts any changes made by the update.
It is a smoke test for concurrent updates.

Now, it expects to hit a certain number of version conflicts
during the updates. This is normal as it is racing the
update-by-query. We have a maximum number of failures we
expect (10) and I'd never seen us come close until
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+5.x+multijob-unix-compatibility/os=sles/495/console

This bumps the max failures from 10 to 50 and improves
logging a bit. If we continue to see this failure then we have
some other issue.

Closes #22938

modules/reindex/src/test/java/org/elasticsearch/index/reindex/UpdateByQueryWhileModifyingTests.java

@@ -39,7 +39,7 @@ import static org.hamcrest.Matchers.equalTo;
  */
 public class UpdateByQueryWhileModifyingTests extends ReindexTestCase {
     private static final int MAX_MUTATIONS = 50;
-    private static final int MAX_ATTEMPTS = 10;
+    private static final int MAX_ATTEMPTS = 50;
 
     public void testUpdateWhileReindexing() throws Exception {
         AtomicReference<String> value = new AtomicReference<>(randomSimpleString(random()));
@@ -83,9 +83,8 @@ public class UpdateByQueryWhileModifyingTests extends ReindexTestCase {
                             throw new RuntimeException(
                                     "Failed to index after [" + MAX_ATTEMPTS + "] attempts. Too many version conflicts!");
                         }
-                        logger.info(
-                                "Caught expected version conflict trying to perform mutation number {} with version {}. Retrying.",
-                                i, get.getVersion());
+                        logger.info("Caught expected version conflict trying to perform mutation number [{}] with version [{}] "
+                                + "on attempt [{}]. Retrying.", i, get.getVersion(), attempts);
                         get = client().prepareGet("test", "test", "test").get();
                     }
                 }