test/ring: fix number of single element enqueue/dequeue

test/ring: fix number of single element enqueue/dequeue

The ring capacity is (RING_SIZE - 1), thus only (RING_SIZE - 1) number of
elements can be enqueued into the ring.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Feifei Wang <feifei.wang2@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Acked-by: Konstantin Ananyev <konstantin.ananyev@intel.com>