Do not hard code ID in client token controller spec

As we do not truncate but rollback the test database it happens
that the ID is alrerady present and will fail the unique index.

spec/controllers/solidus_paypal_braintree/client_tokens_controller_spec.rb

@@ -25,16 +25,12 @@
       end
 
       context 'with a payment method id' do
-        before do
-          create_gateway id: 3
-        end
-
         subject(:response) do
-          post :create, params: { token: user.spree_api_key, payment_method_id: 3 }
+          post :create, params: { token: user.spree_api_key, payment_method_id: gateway.id }
         end
 
         it 'uses the selected gateway' do
-          expect(json["payment_method_id"]).to eq 3
+          expect(json["payment_method_id"]).to eq gateway.id
         end
       end
     end