Try and make this more consistent

Author: nik9000

x-pack/plugin/esql/qa/server/single-node/src/javaRestTest/java/org/elasticsearch/xpack/esql/qa/single_node/EsqlPartitioningIT.java

@@ -179,6 +179,16 @@ private void setupIndex(String index, int docs) throws IOException {
         if (exists.getStatusLine().getStatusCode() == 200) {
             return;
         }
+        Request create = new Request("PUT", index);
+        create.setJsonEntity("""
+            {
+              "settings": {
+                "index": {
+                  "number_of_shards": 1
+                }
+              }
+            }""");  // Use a single shard to get consistent results.
+        client().performRequest(create);
         StringBuilder bulk = new StringBuilder();
         for (int d = 0; d < docs; d++) {
             bulk.append("{\"index\":{}}\n");