core: fix hang in test

test/unit/vagrant/action/builtin/box_add_test.rb

@@ -244,10 +244,13 @@ describe Vagrant::Action::Builtin::BoxAdd do
         box_collection.should_receive(:add).never
         app.should_receive(:call).never
 
+        url = "http://127.0.0.1:#{port}"
+        with_temp_env("VAGRANT_SERVER_URL" => url) do
           expect { subject.call(env) }.
             to raise_error(Vagrant::Errors::BoxAddShortNotFound)
         end
       end
+    end
 
     it "adds the latest version of a box with only one provider" do
       box_path = iso_env.box2_file(:virtualbox)