Added getLocale()

Added locale to the test user.

**Note: This change depends on the extension of `Sulu\Component\Security\UserInterface`**

- [ ] finish the code
- [ ] gather feedback for my changes


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none 
| Doc           | none