[General] Consider SymfonyDependencyInjectionTest

[craigh]

https://github.com/matthiasnoback/SymfonyDependencyInjectionTest#standard-test-for-unobtrusiveness

might be useful to help write tests for HookBundle and some other areas.

[craigh]

ok to put off to 4.0 for me.

[Guite]

closing as unneeded