Testing fixes and improvements

**TODO**
- [x] Reactivates [WebTests](https://github.com/symfony-cmf/RoutingBundle/tree/master/Tests/WebTest)
- [x] Reactivates / implement skipped ones
- [x] Adds tests for the sonata extensions
- [x] Fixes FrontendLinkExtension compatibility with Symfony 2.6

Part of the discussion in #275... 

**PR-Doc**

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | maybe partially #232 
| License       | MIT
| Doc PR        |