Add test assertion for response instance

[robbieaverill]

This prevents middlewares that return null (like the example delegate in this test) from killing a testsuite

Example error without this:

Fatal error: Call to a member function getHeader() on null in /Users/robbieaverill/dev/releases/release-cwp_cwp-recipe-kitchen-sink-2.0.0-rc4/vendor/silverstripe/framework/tests/php/Control/DirectorTest.php on line 596

Also adds some config resets to ensure global state pollution doesn't affect the tests when running in a CWP 2.0 context