added a small test for process.env.WEBPACK_DEV_SERVER

Author: donaldpipowitch

test/cli.test.js

@@ -9,6 +9,15 @@ const execa = require('execa');
 const runDevServer = require('./helpers/run-webpack-dev-server');
 
 describe('CLI', () => {
+  it('process.env.WEBPACK_DEV_SERVER', (done) => {
+    runDevServer()
+      .then(() => {
+        assert(process.env.WEBPACK_DEV_SERVER);
+        done();
+      })
+      .catch(done);
+  }).timeout(18000);
+
   it('--progress', (done) => {
     runDevServer('--progress')
       .then((output) => {