Helper function to initialize authorize.php and load it via drupalGet().
Initializing authorize.php needs to happen in the child Drupal installation, not the parent. So, we visit a menu callback provided by system_test.module which calls system_authorized_init() to initialize the $_SESSION inside the test site, not the framework site. This callback redirects to authorize.php when it's done initializing.
function drupalGetAuthorizePHP($page_title = 'system-test-auth') {
$this
->drupalGet('system-test/authorize-init/' . $page_title);
}