test: remove t_sqe_prep_cmd()

t_sqe_prep_cmd() does the same thing as the recently added liburing
function io_uring_prep_uring_cmd(). Switch to io_uring_prep_uring_cmd()
to provide coverage of the real library function.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>