Adapt test cases to pytest-asyncio 1.0 compatibility

[pastalian]

• Remove deprecated event_loop fixture
  https://pytest-asyncio.readthedocs.io/en/stable/reference/changelog.html#removed
• Drop *_without_event_loop tests
  These incompatible tests (*1) are no longer needed since the underlying code has already been removed (introduced in a460156, removed in 5d714cc).

*1: asyncio.get_event_loop() now raises a RuntimeError in Python 3.14
when no loop exists.
https://docs.python.org/3.14/whatsnew/3.14.html#id7

_____________________________________________________________________________________________ ERROR at setup of test_on_exception_coro_cancelling _____________________________________________________________________________________________
file /var/tmp/portage/dev-python/backoff-2.2.1/work/backoff-2.2.1/tests/test_backoff_async.py, line 667
  @pytest.mark.asyncio
  async def test_on_exception_coro_cancelling(event_loop):
      sleep_started_event = asyncio.Event()

      @backoff.on_predicate(backoff.expo)
      async def coro():
          sleep_started_event.set()

          try:
              await asyncio.sleep(10)
          except asyncio.CancelledError:
              return True

          return False

      task = event_loop.create_task(coro())

      await sleep_started_event.wait()

      task.cancel()

      assert (await task)
E       fixture 'event_loop' not found
>       available fixtures: _class_event_loop, _function_event_loop, _module_event_loop, _package_event_loop, _session_event_loop, cache, capfd, capfdbinary, caplog, capsys, capsysbinary, capteesys, doctest_namespace, event_loop_policy, monkeypatch, pytestconfig, record_property, record_testsuite_property, record_xml_attribute, recwarn, tmp_path, tmp_path_factory, tmpdir, tmpdir_factory, unused_tcp_port, unused_tcp_port_factory, unused_udp_port, unused_udp_port_factory
>       use 'pytest --fixtures [testpath]' for help on them.

__________________________________________________________________________________________ test_on_predicate_on_regular_function_without_event_loop ___________________________________________________________________________________________

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x7f1ab5f52200>

    def test_on_predicate_on_regular_function_without_event_loop(monkeypatch):
        monkeypatch.setattr('time.sleep', lambda x: None)
    
        # Set default event loop to None.
>       loop = asyncio.get_event_loop()
               ^^^^^^^^^^^^^^^^^^^^^^^^

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x7f1ab5f52200>

tests/test_backoff_async.py:695: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <asyncio.unix_events._UnixDefaultEventLoopPolicy object at 0x7f1ab598e660>

    def get_event_loop(self):
        """Get the event loop for the current context.
    
        Returns an instance of EventLoop or raises an exception.
        """
        if self._local._loop is None:
>           raise RuntimeError('There is no current event loop in thread %r.'
                               % threading.current_thread().name)
E           RuntimeError: There is no current event loop in thread 'MainThread'.

self       = <asyncio.unix_events._UnixDefaultEventLoopPolicy object at 0x7f1ab598e660>

/usr/lib/python3.14/asyncio/events.py:718: RuntimeError
__________________________________________________________________________________________ test_on_exception_on_regular_function_without_event_loop ___________________________________________________________________________________________

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x7f1ab5846190>

    def test_on_exception_on_regular_function_without_event_loop(monkeypatch):
        monkeypatch.setattr('time.sleep', lambda x: None)
    
        # Set default event loop to None.
>       loop = asyncio.get_event_loop()
               ^^^^^^^^^^^^^^^^^^^^^^^^

monkeypatch = <_pytest.monkeypatch.MonkeyPatch object at 0x7f1ab5846190>

tests/test_backoff_async.py:719: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self = <asyncio.unix_events._UnixDefaultEventLoopPolicy object at 0x7f1ab598e660>

    def get_event_loop(self):
        """Get the event loop for the current context.
    
        Returns an instance of EventLoop or raises an exception.
        """
        if self._local._loop is None:
>           raise RuntimeError('There is no current event loop in thread %r.'
                               % threading.current_thread().name)
E           RuntimeError: There is no current event loop in thread 'MainThread'.

self       = <asyncio.unix_events._UnixDefaultEventLoopPolicy object at 0x7f1ab598e660>

/usr/lib/python3.14/asyncio/events.py:718: RuntimeError