Separate autoscaling REST test cases

A couple of the autoscaling REST tests combine multiple tests into a
single REST test. This commit separates them in to single tests.

x-pack/plugin/autoscaling/qa/rest/src/test/resources/rest-api-spec/test/autoscaling/delete_autoscaling_policy.yml

@@ -16,6 +16,8 @@
 
   # TODO: add validation that the policy is removed after we have a get policy API
 
+---
+"Test delete non-existent policy":
   - do:
       catch: bad_request
       autoscaling.delete_autoscaling_policy:

x-pack/plugin/autoscaling/qa/rest/src/test/resources/rest-api-spec/test/autoscaling/put_autoscaling_policy.yml

@@ -10,6 +10,8 @@
 
   - match: { "acknowledged": true }
 
+---
+"Test put autoscaling policy with non-existent decider":
   - do:
       catch: bad_request
       autoscaling.put_autoscaling_policy: