Adding capability to the test helper to load more than one node at once

When splitting the credentials for the swarm/foursquare web nodes (https://github.com/node-red/node-red-web-nodes/issues/41) I need to be able to test the swarm node by also loading the foursquare-credentials node. This requires an update to the test helper class in the main node-red stream to enable the loading of more than one node.