lib/utils: add unit test for async-signal-safe assert()

Don't try to test async-signal-safety, only that
NBD_INTERNAL_FORK_SAFE_ASSERT() works similarly to assert():

- it prints diagnostics to stderr,

- it calls abort().

Some unfortunate gymnastics are necessary to avoid littering the system
with unwanted core dumps.

Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20230315110157.357958-4-lersek@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>