[WIP] 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        | 
i just hate ` @param string $action The action` style comments. rather really miss them so its visible. but indeed it would make a lot of sense to explain for example what that string means and what it may be.

oh, or as this actually is implementing an interface method, the right thing would be `{@inheritDoc}` first and then whatever specific doc we have for this class ("Adds a tab linking to this document in the frontend.")