Cosmetic improvements in in test/test_weakref.py

- correct the call-signature of the event-method (did not trigger an
  exception because the event was not actually called)
- demonstrate that object deletion can have more subtle causes than
  explicit deletion using `del`
Test object stay-alive mechanism during event handler execution

Refer: 4f9d16d56e06651397962bab7a13b8fc2779e93d