tests/unit: remove block layer code from test-nested-aio-poll

tests/unit: remove block layer code from test-nested-aio-poll

GCC is reporting a NULL pointer dereference when compiling aio_wait_kick()
with LTO.

The issue is that test-nested-aio-poll.c does not call qemu_init_main_loop().
It doesn't _need_ to because it never calls AIO_WAIT_WHILE(), but it seems
that LTO does not do enough dead-code elimination to catch that.

Fortunately aio_wait_kick() is only used in few places, and only in block
layer or system emulation code; and this test only needs the core event loop
functionality.  It does not even need iothreads.   So remove everything that
calls aio_wait_kick(), which is nice for coverage compared to adding the call
to qemu_init_main_loop().

Reported-by: Richard W.M. Jones <rjones@redhat.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2434
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>