Add assertion that num_reduce_phases >= 1

Author: benchaplin

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search/120_batch_reduce_size.yml

@@ -48,6 +48,7 @@ setup:
  batched_reduce_size: 2
  body: { "size" : 0, "aggs" : { "str_terms" : { "terms" : { "field" : "str" } } } }
 
+ - gte: { num_reduce_phases: 1 }
  - match: { hits.total: 3 }
  - length: { aggregations.str_terms.buckets: 2 }
  - match: { aggregations.str_terms.buckets.0.key: "abc" }