plugin: Test endpoints via supertest

Unfortunately we can't use node-mocks-http to test a express.Router
that has async routes. See https://github.com/howardabrams/node-mocks-http/issues/225

router will just return undefined if the executing handler is async and
so the test will have no way to wait for it to complete. Thus, we have
to use supertest which starts an actual HTTP server in the background
and uses a HTTP client to send requests.