Fixing ValuesSourceReaderOperatorTests by reducing merge at once

Author: benwtrent

x-pack/plugin/esql/compute/src/test/java/org/elasticsearch/compute/lucene/read/ValuesSourceReaderOperatorTests.java

@@ -401,7 +401,8 @@ private IndexReader initIndexLongField(Directory directory, int size, int commit
         try (
             IndexWriter writer = new IndexWriter(
                 directory,
-                newIndexWriterConfig().setMergePolicy(new TieredMergePolicy()).setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH)
+                newIndexWriterConfig().setMergePolicy(new TieredMergePolicy().setMaxMergeAtOnce(5))
+                    .setMaxBufferedDocs(IndexWriterConfig.DISABLE_AUTO_FLUSH)
             )
         ) {
             for (int d = 0; d < size; d++) {