Use let_it_be for policy tests

Also change type to UserNamespace to match class

spec/policies/namespaces/user_namespace_policy_spec.rb

@@ -3,10 +3,10 @@
 require 'spec_helper'
 
 RSpec.describe Namespaces::UserNamespacePolicy do
-  let(:user) { create(:user) }
-  let(:owner) { create(:user) }
-  let(:admin) { create(:admin) }
-  let(:namespace) { create(:namespace, owner: owner) }
+  let_it_be(:user) { create(:user) }
+  let_it_be(:owner) { create(:user) }
+  let_it_be(:admin) { create(:admin) }
+  let_it_be(:namespace) { create(:user_namespace, owner: owner) }
 
   let(:owner_permissions) { [:owner_access, :create_projects, :admin_namespace, :read_namespace, :read_statistics, :transfer_projects, :create_package_settings, :read_package_settings] }