Update test server

scripts/test-helper.js

@@ -8,10 +8,19 @@ const app = express();
 let devServer = null;
 
 global.startServer = (done) => {
+  if (devServer === null) {
     app.use('/', express.static(path.resolve(__dirname, '..')));
     devServer = app.listen(3000, done);
+  } else {
+    done();
+  }
 };
 
 global.stopServer = (done) => {
+  if (devServer !== null) {
     devServer.close(done);
+    devServer = null;
+  } else {
+    done();
+  }
 };