test/cq-peek-batch-mixed: add peek-batch test for mixed CQE rings

Test io_uring_peek_batch_cqe() on an IORING_SETUP_CQE_MIXED ring:
mixed 16b/32b batches must return pointers to CQE starts, and the
skip entry the kernel posts at ring wrap must stop the batch when
mid-window and be consumed when at the CQ head.

Signed-off-by: Jens Axboe <axboe@kernel.dk>