Hotfix #3046 - set /dev/urandom as entropy file for Session

I removed the is_file() check in setEntropyFile() of Zend\Session\Config\StandardConfig in order to accept /dev/urandom, /dev/random or /dev/arandom if it is available.
I addded a unit test for that case and removed the dir tests.
See https://github.com/zendframework/zf2/issues/3046