Test: Switch painless test to 1 shard

We *think* that #28600 is caused by warnings not being collected during
one of the fan out phases of search but we're not 100% sure how this is
happening. This commit drops the number of shards used for the test to 1
so there *isn't* a fan out phase. If this makes the issue go away we'll
have more information.

Author: nik9000

modules/lang-painless/src/test/resources/rest-api-spec/test/painless/50_script_doc_values.yml

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