Fix UserPermissions unit test

Without having a current user, the permission would be `false` even if the logic test in `canChangeRole()` would not work, thus making the test useless

tests/Cms/Users/UserPermissionsTest.php

@@ -131,6 +131,8 @@ public function testChangeSingleRole()
 			]
 		]);
 
+		$app->impersonate('kirby');
+
 		$user  = new User(['email' => '[email protected]']);
 		$perms = $user->permissions();