Not able to run test for individual apps

**I'm submitting a ...** (check one with "x")
```
[X ] bug report => search github for a similar issue or PR before submitting
[ ] feature request
[ ] support request => Please do not submit support request here, instead see use [gitter](https://gitter.im/mgechev/angular2-seed) or [stackoverflow](https://stackoverflow.com/questions/tagged/angular2)
```

**Current behavior**

npm test does not take app parameter.. we have 2 applications using same angular seed.
if I run npm test -- --app sd, it does not run tests for sd application

**Expected behavior**

Like the way, we can start and build individual applications by passing --app parameter, npm test with app parameter should run
**Minimal reproduction of the problem with instructions**

execute npm test -- --app

**What is the motivation / use case for changing the behavior?**

In a distributed team environment, developers should be able to run tests on individual application
**Please tell us about your environment:**

Windows 10

* **Angular Seed Version:** `aaaaf75`

* **Node:** `node --version` =

Start by reproducing `npm test -- --app ` in the Angular seed project and inspect the test command configuration that handles the `--app` argument. Done means the command runs tests only for the selected application while existing test behavior remains intact.

Written by the indexing model from the issue text.