test: reduce replicas for indices synced flush test

We have to make sure all shards are started to know the synced flush will hit them all. Shards that are still initializing during the sync flush may be missed and confuse the stats call

rest-api-spec/test/indices.flush/10_basic.yaml

@@ -3,10 +3,14 @@
     - do:
        indices.create:
           index: testing
+          body:
+            settings:
+              index:
+                number_of_replicas: 0
 
     - do:
        cluster.health:
-          wait_for_status: yellow
+          wait_for_status: green
     - do:
        indices.flush_synced:
           index: testing