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>

tests/unit/meson.build

@@ -116,7 +116,7 @@ if have_block
   if host_os != 'windows'
     tests += {
       'test-image-locking': [testblock],
-      'test-nested-aio-poll': [testblock],
+      'test-nested-aio-poll': [],
     }
   endif
   if config_host_data.get('CONFIG_REPLICATION')