Correct test TestParentsChildrenBlockJoinQuery.testExplain (#15042)

Author: benwtrent

lucene/join/src/test/org/apache/lucene/search/join/TestParentsChildrenBlockJoinQuery.java

@@ -27,6 +27,7 @@
 import org.apache.lucene.document.StoredField;
 import org.apache.lucene.document.StringField;
 import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.NoMergePolicy;
 import org.apache.lucene.index.Term;
 import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.BooleanQuery;
@@ -353,7 +354,7 @@ public void testExplain() throws Exception {
     Directory dir = newDirectory();
     final RandomIndexWriter writer =
         new RandomIndexWriter(
-            random(), dir, newIndexWriterConfig().setMergePolicy(newMergePolicy(random(), false)));
+            random(), dir, newIndexWriterConfig().setMergePolicy(NoMergePolicy.INSTANCE));
 
     // Add documents
     List<Document> docs = new ArrayList<>();