ck_ring: Unit test coverage for ck_ring_trydequeue.

regressions/ck_ring/validate/ck_ring_spmc.c

@@ -82,7 +82,11 @@ test_spmc(void *c)
 			struct entry *o;
 
 			/* Keep trying until we encounter at least one node. */
-			while (ck_ring_dequeue_spmc(&ring_spmc, &o) == false);
+			if (j & 1) {
+				while (ck_ring_dequeue_spmc(&ring_spmc, &o) == false);
+			} else {
+				while (ck_ring_trydequeue_spmc(&ring_spmc, &o) == false);
+			}
 
 			observed++;
 			if (o->value < 0

regressions/ck_ring/validate/ck_ring_spmc_template.c

@@ -83,7 +83,13 @@ test_spmc(void *c)
 			struct entry o;
 
 			/* Keep trying until we encounter at least one node. */
-			while (CK_RING_DEQUEUE_SPMC(spmc_ring, &ring_spmc, &o) == false);
+			if (j & 1) {
+				while (CK_RING_DEQUEUE_SPMC(spmc_ring,
+					  &ring_spmc, &o) == false);
+			} else {
+				while (CK_RING_TRYDEQUEUE_SPMC(spmc_ring,
+					  &ring_spmc, &o) == false);
+			}
 
 			observed++;
 			if (o.value < 0