gh-94808: [coverage] Add an asynchronous generator test

[zhanpon]

• Issue: Metabug: Improving C-level coverage #94808

The test covers the following code path:

cpython/Objects/genobject.c

Lines 1780 to 1785 in ff54dd9

	if (o->ags_gen->ag_running_async) {
	PyErr_SetString(
	PyExc_RuntimeError,
	"anext(): asynchronous generator is already running");
	return NULL;
	}

[mdboom]

Thanks for improving our test coverage! Can you point to the specific lines that this adds coverage for?

[zhanpon]

@mdboom Thank you for the comment! I've added it to the PR description.

[kumaraditya303]

Thanks.