Add "broken" block inheritance test case

So, as the test shows, I would expect `{{ block('xxx') }}` to use inheritance.

cc @Stof / https://github.com/KnpLabs/KnpMenu/pull/119

```
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'Element:<div>Element:<div></div>Element:<div></div></div>'
+'Element:<div><div></div><div></div></div>'
```