Fix incremental reduce randomization in base tests cases

We can and should randomly reduce down to a single result before
we passing the aggs to the final reduce. This commit changes the logic
to do that and ensures we don't trip the assertions the previous imple tripped.

Relates to #23253

Author: s1monw

core/src/test/java/org/elasticsearch/search/aggregations/AggregatorTestCase.java

@@ -180,9 +180,9 @@ protected <A extends InternalAggregation, C extends Aggregator> A searchAndReduc
             if (aggs.isEmpty()) {
                 return null;
             } else {
-                if (aggs.size() > 2 && randomBoolean()) {
+                if (randomBoolean()) {
                     // sometimes do an incremental reduce
-                    List<InternalAggregation> internalAggregations = randomSubsetOf(randomIntBetween(2, aggs.size()-1), aggs);
+                    List<InternalAggregation> internalAggregations = randomSubsetOf(randomIntBetween(1, aggs.size()), aggs);
                     A internalAgg = (A) aggs.get(0).doReduce(internalAggregations,
                         new InternalAggregation.ReduceContext(root.context().bigArrays(), null, false));
                     aggs.removeAll(internalAggregations);