Added a test to ensure consistent behavior accross drivers for 500

This ensures that drivers don't throw an exception if the page is a 500.
An exception in visit() is allowed if the HTTP request cannot be performed, but not if the tested app returns a valid error response.

Refs https://github.com/Behat/Mink/issues/494
I don't know how to write a test to show that an exception is thrown if the request itself cannot be performed. This will depend on the driver, and simulating network failure is hard