HSEARCH-3881 Reduce the init batch size in SingleFieldAggregationBaseIT

To avoid errors caused by too many requests being sent to Elasticsearch
simultaneously.

integrationtest/backend/tck/src/main/java/org/hibernate/search/integrationtest/backend/tck/search/aggregation/SingleFieldAggregationBaseIT.java

@@ -60,7 +60,7 @@
 @RunWith(Parameterized.class)
 public class SingleFieldAggregationBaseIT<F> {
 
-	private static final int INIT_BATCH_SIZE = 1000;
+	private static final int INIT_BATCH_SIZE = 500;
 
 	private static final String AGGREGATION_NAME = "aggregationName";