Use LogDocMergePolicy in DiversifiedSamplerTests#testDiversifiedSampl…

…er (elastic#107826)

Similar to tother cases, the addition of a new merge policy that reverse the order of the documents in lucene causes 
this test to fail in edge cases. To avoid randomisation we hardcode the merge policy to LogDocMergePolicy.

server/src/test/java/org/elasticsearch/search/aggregations/bucket/sampler/DiversifiedSamplerTests.java

@@ -16,9 +16,13 @@
 import org.apache.lucene.document.TextField;
 import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexReader;
+import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.index.LogDocMergePolicy;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.store.Directory;
+import org.apache.lucene.tests.analysis.MockAnalyzer;
 import org.apache.lucene.tests.index.RandomIndexWriter;
+import org.apache.lucene.tests.util.LuceneTestCase;
 import org.apache.lucene.util.BytesRef;
 import org.elasticsearch.common.lucene.search.function.FieldValueFactorFunction;
 import org.elasticsearch.common.lucene.search.function.FunctionScoreQuery;
@@ -85,7 +89,9 @@ private void writeBooks(RandomIndexWriter iw) throws IOException {
 
     public void testDiversifiedSampler() throws Exception {
         Directory directory = newDirectory();
-        RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory);
+        IndexWriterConfig iwc = LuceneTestCase.newIndexWriterConfig(random(), new MockAnalyzer(random()));
+        iwc.setMergePolicy(new LogDocMergePolicy());
+        RandomIndexWriter indexWriter = new RandomIndexWriter(random(), directory, iwc);
         MappedFieldType genreFieldType = new KeywordFieldMapper.KeywordFieldType("genre");
         writeBooks(indexWriter);
         indexWriter.close();