Fix test failures with the new constant_keyword field. (#53153)

This test failed because YAML tests randomly install an index template
that updates the default number of shards to 2.

Closes #53131

Author: jpountz

x-pack/plugin/src/test/resources/rest-api-spec/test/constant_keyword/10_basic.yml

@@ -8,6 +8,8 @@ setup:
       indices.create:
         index:  test1
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               foo:
@@ -18,6 +20,8 @@ setup:
       indices.create:
         index:  test2
         body:
+          settings:
+            number_of_shards: 1
           mappings:
             properties:
               foo: