Pysim should reject async generators

[cr1901]

While converting some testbenches from generator-based to async-based for pytest-amaranth-sim, I accidentally forgot to remove a yield on one of them:

@pytest.fixture(params=[pytest.param(MulTbArgs())], ids=mul_tb_id)
def mul_tb(mod, request):
    """The multiplier testbench proper. Receives arguments via request."""
    async def testbench(s):
        m = mod

        if m.registered:
            await s.tick()
        else:
            await s.delay(0.1)

        s.set(m.a, request.param.a_in)
        s.set(m.b, request.param.b_in)

        if m.registered:
            await s.tick()
        else:
            await s.delay(0.1)

        # assert s.get(m.o) == request.param.o_out
        assert (yield m.o) == request.param.o_out

    return testbench

Running a simulation with this testbench currently fails with an unhelpful TypeError: mul_tb.<locals>.testbench() missing 1 required positional argument: 's'.

In reality, what happened is "forgetting to remove a yield turns the testbench into an async generator, and Simulator.add_process treats it as as a plain-old-generator". Plain-old-generator testbenches don't expect any input arguments, but async-based testbenches do expect a sim argument, which is missing which that testbench is run. Agreed-upon fix is to not allow async generators, which in and of itself will give a better error message.