[TEST] ensure cluster is stable before running testReindex (elastic#122589) (elastic#122621)

Unable to reproduce however the coordinator node that is meant to route
the write requests might've not been ready to do so in due time.
This PR adds an ensureStableCluster in the test setup method.

Fixes elastic#120605

Author: andreidan

modules/reindex/src/internalClusterTest/java/org/elasticsearch/index/reindex/BulkByScrollUsesAllScrollDocumentsAfterConflictsIntegTests.java

@@ -97,6 +97,7 @@ public void setUpCluster() {
         // Use a single thread pool for writes so we can enforce a consistent ordering
         internalCluster().startDataOnlyNode(Settings.builder().put("thread_pool.write.size", 1).build());
         internalCluster().startCoordinatingOnlyNode(Settings.EMPTY);
+        ensureStableCluster(3);
     }
 
     public void testUpdateByQuery() throws Exception {

muted-tests.yml

@@ -196,9 +196,6 @@ tests:
   issue: https://github.com/elastic/elasticsearch/issues/120482
 - class: org.elasticsearch.xpack.sql.expression.function.scalar.datetime.DateTimeToCharProcessorTests
   issue: https://github.com/elastic/elasticsearch/issues/120575
-- class: org.elasticsearch.index.reindex.BulkByScrollUsesAllScrollDocumentsAfterConflictsIntegTests
-  method: testReindex
-  issue: https://github.com/elastic/elasticsearch/issues/120605
 - class: org.elasticsearch.xpack.inference.DefaultEndPointsIT
   method: testMultipleInferencesTriggeringDownloadAndDeploy
   issue: https://github.com/elastic/elasticsearch/issues/120668