Add a test to check FormElementManager has ServiceLocator in Mvc #3923

Relative to #3923

In case instances made in AbstractPluginManager implement ServiceLocatorAwareInterface, these have AbstractPluginManager as ServiceLocator.
Is it designed behavior?

This test indicates that we can get the Application's ServiceLocator from  AbstractPluginManager::getServiceLocator in the Mvc scope.

notes:
https://github.com/zendframework/zf2/issues/4464#issuecomment-17797941