gh-128404: Run TestGetAsyncGenState without asyncio

[graingert]

inspect.getasyncgenstate() and getasyncgenlocals() are pure introspection: none of the awaits in these tests suspend, so the class does not need an event loop and does not need to be skipped when there is no working socket.

Drive the coroutines with support.run_no_yield_async_fn() instead, as the neighbouring TestGetCoroutineState already does, using the same _async_test decorator as test_contextlib_async. This also drops the last use of asyncio in the module.

• Issue: Remove un-necessary socket requirements in async tests #128404

[johnslavik]

Maybe we should move this helper to somewhere in test.support? #155127 will also use it.
We'll have 3 test modules using @_async_test.

[johnslavik]

Oh, right. I rewrote the decorator as _async_test = partial(partial, run_yielding_async_fn). Who doesn't love partial partials.

[graingert]

it needs functools.wraps to copy in flags such as __unittest_skip__ = True and
__unittest_skip_why__ = reason

edit: I suppose you can put the skipIf/skip decorator on top of the @_async_test decorator

Oh, right. I rewrote the decorator as _async_test = partial(partial, run_yielding_async_fn). Who doesn't love partial partials.

[graingert]

my @_async_test uses run_no_yield_async_fn but your @_async_test uses run_yielding_async_fn so I think they're different enough to be defined per module