test: use a single primary shard to workaround an edge case with the phrase suggester

Relates to elastic/x-pack-elasticsearch#2804

Original commit: elastic/x-pack-elasticsearch@afd028faf7

plugin/src/test/java/org/elasticsearch/integration/DocumentLevelSecurityTests.java

@@ -956,6 +956,7 @@ public class DocumentLevelSecurityTests extends SecurityIntegTestCase {
 
     public void testSuggesters() throws Exception {
         assertAcked(client().admin().indices().prepareCreate("test")
+                .setSettings(Settings.builder().put("index.number_of_shards", 1))
                 .addMapping("type1", "field1", "type=text", "suggest_field1", "type=text", "suggest_field2", "type=completion")
         );