Use a default host for fixtures
Use a default host for fixtures

This makes it much simpler to use the webserver script, while still allowing
overwrites. Making drivers use this URL by default in phpunit.xml.dist would
not allow to overwrite it through env variables, as the phpunit config file
wins over the environment variables.

minkphp/driver-testsuite,minkphp/driver-testsuite,minkphp/driver-testsuite