Use newIndexSearcher() instead of newSearcher() (#45248)

`newSearcher()` from lucene can randomly choose index readers which
are not compatible with our tests, like ParallelCompositeReader.
The `newIndexSearcher()` method on AggregatorTestCase is a wrapper
similar to newSearcher but compatible with our tests

server/src/test/java/org/elasticsearch/search/aggregations/pipeline/BucketScriptAggregatorTests.java

@@ -111,7 +111,7 @@ public class BucketScriptAggregatorTests extends AggregatorTestCase {
             indexWriter.close();
 
             try (IndexReader indexReader = DirectoryReader.open(directory)) {
-                IndexSearcher indexSearcher = newSearcher(indexReader, true, true);
+                IndexSearcher indexSearcher = newIndexSearcher(indexReader);
 
                 InternalFilters filters;
                 filters = searchAndReduce(indexSearcher, query, aggregationBuilder, fieldType);