Fix broken yml test for combined_fields. (opensearch-project#18971)

This test breaks with a repeatable seed because we randomly
set shard count above 1 which will impact scoring.  Fixing it
by setting shard count to 1 instead of using dfs query then fetch.

Signed-off-by: Marc Handalian <marc.handalian@gmail.com>

Author: mch2

rest-api-spec/src/main/resources/rest-api-spec/test/search/400_combined_fields.yml

@@ -3,6 +3,8 @@ setup:
       indices.create:
         index: cf_test
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               headline: