The new web notifications tests in http/tests/notifications use an updated API for managing web notifications:
// Web notifications support
void grantWebNotificationPermission(in DOMString origin);
void denyWebNotificationPermission(in DOMString origin);
void simulateWebNotificationClick(in object notification);
These need to be implemented in order for the tests to work. (grantWebNotificationPermission is already ported from grantDesktopNotificationPermission).
It seems to work fine in WTR, the implementation of most of the methods can be found in bug #107449