Wait for cluster to be green in 40_get_secrets (#117217)

Now that fast refresh searches go to search nodes instead of index
nodes, the YAML test for `fleet.get_secret` sometimes fails if
the shard isn't available yet on the search node before the request
arrives.

Addresses elasticsearch-serverless #3159, which can unmute the test
once this is available.

Author: bcully

x-pack/plugin/fleet/qa/rest/src/yamlRestTest/resources/rest-api-spec/test/fleet/40_secrets_get.yml

@@ -3,6 +3,12 @@
       fleet.post_secret:
         body: '{"value": "test secret"}'
   - set:  { id: id }
+  # search node needs to be available for fleet.get_secret to work in stateless.
+  # The `.fleet-secrets` index is created on demand, and its search replica starts out unassigned,
+  # so wait_for_no_uninitialized_shards can miss it.
+  - do:
+      cluster.health:
+        wait_for_active_shards: all
   - do:
       fleet.get_secret:
         id: $id