[CI] Use a single shard for synonym REST tests, to ensure score ordering

is stable

modules/analysis-common/src/test/resources/rest-api-spec/test/search.query/60_synonym_graph.yml

@@ -4,6 +4,8 @@ setup:
         index: test
         body:
           settings:
+            index:
+              number_of_shards: 1     # keep scoring stable
             analysis:
               filter:
                 syns: