Suspicion of flaky test: test_cancelscope_propagation_when_abandoned

### Things to check first

- [X] I have searched the existing issues and didn't find my bug already reported there

- [X] I have checked that my bug is still present in the latest release

### AnyIO version

4.1.0

### Python version

3.10

### What happened?

I found a test flakiness/race in:

https://github.com/NixOS/nixpkgs/pull/281858#issuecomment-1908329079

```
=================================== FAILURES ===================================
______________ test_cancelscope_propagation_when_abandoned[trio] _______________
[gw31] linux -- Python 3.10.13 /nix/store/8np01kr8qpm49m2gzjbwcjr68f162b9x-python3-3.10.13/bin/python3.10
/nix/store/r4mhvx3r2pigizgch91dvx4s7kf10baw-python3.10-pytest-7.4.3/lib/python3.10/site-packages/_pytest/runner.py:341: in from_call
result: Optional[TResult] = func()
/nix/store/r4mhvx3r2pigizgch91dvx4s7kf10baw-python3.10-pytest-7.4.3/lib/python3.10/site-packages/_pytest/runner.py:262: in
lambda: ihook(item=item, **kwds), when=when, reraise=reraise
/nix/store/wmx47qf64ymxicnjiw05p2y54r998h5i-python3.10-pluggy-1.3.0/lib/python3.10/site-packages/pluggy/_hooks.py:493: in __call__
return self._hookexec(self.name, self._hookimpls, kwargs, firstresult)
/nix/store/wmx47qf64ymxicnjiw05p2y54r998h5i-python3.10-pluggy-1.3.0/lib/python3.10/site-packages/pluggy/_manager.py:115: in _hookexec
return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
/nix/store/r4mhvx3r2pigizgch91dvx4s7kf10baw-python3.10-pytest-7.4.3/lib/python3.10/site-packages/_pytest/threadexception.py:83: in pytest_runtest_call
yield from thread_exception_runtest_hook()
/nix/store/r4mhvx3r2pigizgch91dvx4s7kf10baw-python3.10-pytest-7.4.3/lib/python3.10/site-packages/_pytest/threadexception.py:73: in thread_exception_runtest_hook
warnings.warn(pytest.PytestUnhandledThreadExceptionWarning(msg))
E pytest.PytestUnhandledThreadExceptionWarning: Exception in thread AnyIO worker thread
E
E Traceback (most recent call last):
E File "/nix/store/8np01kr8qpm49m2gzjbwcjr68f162b9x-python3-3.10.13/lib/python3.10/threading.py", line 1016, in _bootstrap_inner
E self.run()
E File "/nix/store/1fdn4d018vfn7ygwaiy451iyp9va54nh-python3.10-anyio-4.1.0/lib/python3.10/site-packages/anyio/_backends/_asyncio.py", line 830, in run
E self.loop.call_soon_threadsafe(
E File "uvloop/loop.pyx", line 1288, in uvloop.loop.Loop.call_soon_threadsafe
E File "uvloop/loop.pyx", line 671, in uvloop.loop.Loop._append_ready_handle
E File "uvloop/loop.pyx", line 703, in uvloop.loop.Loop._check_closed
E RuntimeError: Event loop is closed
```

Re-running made the test pass.

### How can we reproduce the bug?

Unclear, probably run the test often enough?