test(secrets): fix flakiness

Signed-off-by: Francesco Noacco <francesco.noacco@secomind.com>

Author: noaccOS

libs/astarte_secrets/test/test_helper.exs

@@ -11,4 +11,8 @@ for module <- modules, do: Mimic.copy(module)
 
 Astarte.Secrets.Config.init()
 
+# fix flakiness due to async tests
+Astarte.Secrets.Core.create_nested_namespace(["fdo_owner_keys", "default_instance"])
+Astarte.Secrets.Core.create_nested_namespace(["fdo_owner_keys", "instance"])
+
 ExUnit.start(capture_log: true)