Fix composite test

Our test for early termination would break if we made many small
segments because none of them would be large enough to trigger the early
termination. This makes sure have only a single segment for these test,
making sure we terminate early.

Closes #62769

server/src/test/java/org/elasticsearch/search/aggregations/bucket/composite/CompositeAggregatorTests.java

@@ -2133,7 +2133,7 @@ public void testEarlyTermination() throws Exception {
             )
         );
 
-        executeTestCase(true, new TermQuery(new Term("foo", "bar")),
+        executeTestCase(true, true, new TermQuery(new Term("foo", "bar")),
             dataset,
             () ->
                 new CompositeAggregationBuilder("name",
@@ -2153,7 +2153,7 @@ public void testEarlyTermination() throws Exception {
         );
 
         // source field and index sorting config have different order
-        executeTestCase(true, new TermQuery(new Term("foo", "bar")),
+        executeTestCase(true, true, new TermQuery(new Term("foo", "bar")),
             dataset,
             () ->
                 new CompositeAggregationBuilder("name",
@@ -2190,7 +2190,7 @@ public void testIndexSortWithDuplicate() throws Exception {
         );
 
         for (SortOrder order : SortOrder.values()) {
-            executeTestCase(true, new MatchAllDocsQuery(),
+            executeTestCase(false, true, new MatchAllDocsQuery(),
                 dataset,
                 () ->
                     new CompositeAggregationBuilder("name",
@@ -2213,7 +2213,7 @@ public void testIndexSortWithDuplicate() throws Exception {
                 }
             );
 
-            executeTestCase(true, new MatchAllDocsQuery(),
+            executeTestCase(false, true, new MatchAllDocsQuery(),
                 dataset,
                 () ->
                     new CompositeAggregationBuilder("name",
@@ -2243,12 +2243,13 @@ private void testSearchCase(List<Query> queries,
                                 Supplier<CompositeAggregationBuilder> create,
                                 Consumer<InternalComposite> verify) throws IOException {
         for (Query query : queries) {
-            executeTestCase(false, query, dataset, create, verify);
-            executeTestCase(true, query, dataset, create, verify);
+            executeTestCase(false, false, query, dataset, create, verify);
+            executeTestCase(false, true, query, dataset, create, verify);
         }
     }
 
-    private void executeTestCase(boolean useIndexSort,
+    private void executeTestCase(boolean forceMerge,
+                                 boolean useIndexSort,
                                  Query query,
                                  List<Map<String, List<Object>>> dataset,
                                  Supplier<CompositeAggregationBuilder> create,
@@ -2273,18 +2274,21 @@ private void executeTestCase(boolean useIndexSort,
                     indexWriter.addDocument(document);
                     id++;
                 }
-                if (rarely()) {
+                if (forceMerge || rarely()) {
+                    // forceMerge randomly or if the collector-per-leaf testing stuff would break the tests.
                     indexWriter.forceMerge(1);
-                }
-                if (dataset.size() > 0) {
-                    int numDeletes = randomIntBetween(1, 25);
-                    for (int i = 0; i < numDeletes; i++) {
-                        id = randomIntBetween(0, dataset.size() - 1);
-                        indexWriter.deleteDocuments(new Term("id", Integer.toString(id)));
-                        document.clear();
-                        addToDocument(id, document, dataset.get(id));
-                        indexWriter.addDocument(document);
+                } else {
+                    if (dataset.size() > 0) {
+                        int numDeletes = randomIntBetween(1, 25);
+                        for (int i = 0; i < numDeletes; i++) {
+                            id = randomIntBetween(0, dataset.size() - 1);
+                            indexWriter.deleteDocuments(new Term("id", Integer.toString(id)));
+                            document.clear();
+                            addToDocument(id, document, dataset.get(id));
+                            indexWriter.addDocument(document);
+                        }
                     }
+
                 }
             }
             try (IndexReader indexReader = DirectoryReader.open(directory)) {