request_var tests fail when other tests using request_var are executed first

All tests in tests/request/request_var.php fail because request_var() calls use the mock request instance created in previous tests. Instead the default request_var behaviour of creating its on instance on use should apply.

What's required for this to work is some way to reset the static instance inside the deprecated request_var function to null for further tests. PHPUnit's global/static scope backup and restore mechanism cannot access static function values.