Test nested event emission

[dshcherb]

I ran into an issue with the current object singularity tracking implementation when writing a test charm where I needed to emit an event in an event handler.

Example: on_start -> emit charm-specific event -> error.

This happens because of the current descriptor pattern implementation that creates new copies of EventsBase descendants by using type(self)(emitter):

....................E............................................
======================================================================
ERROR: test_weak_observer (test.test_framework.TestFramework)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/ubuntu/src/juju/operator/test/test_framework.py", line 386, in test_weak_observer
    pub.on.foo.emit()
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 172, in emit
    framework._emit(event)
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 562, in _emit
    self._reemit(event_path)
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 597, in _reemit
    custom_handler(event)
  File "/home/ubuntu/src/juju/operator/test/test_framework.py", line 366, in on_foo
    self.on.bar.emit()
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 230, in __get__
    return type(self)(emitter)
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 223, in __init__
    super().__init__(parent, key)
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 201, in __init__
    self.framework._track(self)
  File "/home/ubuntu/src/juju/operator/ops/framework.py", line 413, in _track
    raise RuntimeError(f"two objects claiming to be {obj.handle.path} have been created")
RuntimeError: two objects claiming to be MyNotifier[1]/on have been created

The issue is also possible to trigger by using something like this:

        for e in (pub.on.foo,
                  pub.on.bar):
            framework.observe(e, obs)

[dshcherb]

#99 should address this.