set a default loop exception_handler that warns (similar to -p unraisablehook)

[graingert]

consider the following test:

import pytest
import asyncio

@pytest.mark.asyncio
async def test_foo():
    def raise_():
        raise Exception

    asyncio.get_running_loop().call_soon(raise_)
    await asyncio.sleep(0)

it passes cleanly.

However if you explicitly collect any errors:

import pytest
import asyncio

@pytest.mark.asyncio
async def test_foo():
    loop = asyncio.get_running_loop()
    old_exception_handler = loop.get_exception_handler()
    exceptions = []

    def exception_handler(*args, **kwargs):
        exceptions.append((args, kwargs))

    loop.set_exception_handler(exception_handler)
    try:
        def raise_():
            raise Exception

        asyncio.get_running_loop().call_soon(raise_)
        await asyncio.sleep(0)
        assert exceptions == []
    finally:
        loop.set_exception_handler(old_exception_handler)

=================================== test session starts ====================================
platform linux -- Python 3.9.2, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: /home/graingert/projects/anyio, configfile: pyproject.toml
plugins: hypothesis-6.2.0, asyncio-0.14.0, anyio-2.1.0.post21+dirty
collected 1 item                                                                           

ham.py F                                                                             [100%]

========================================= FAILURES =========================================
_________________________________________ test_foo _________________________________________
ham.py:20: in test_foo
    assert exceptions == []
E   AssertionError: assert [((<_UnixSele...py:15'}), {})] == []
E     Left contains one more item: ((<_UnixSelectorEventLoop running=True closed=False debug=False>, {'exception': Exception(), 'handle': <Handle test_fo...y:15>, 'message': 'Exception in callback test_foo.<locals>.raise_() at /home/graingert/projects/anyio/ham.py:15'}), {})
E     Use -v to get the full diff
==================================== 1 failed in 0.04s ====================================

[HMaker]

I have a similar case, exceptions in background tasks pass silently. It would be good if background exceptions were checked at test end and re-raised there.