t-prio-queue: shorten array index message

If we get an unexpected result, the prio-queue unit test reports it like
this:

 # check "result[j++] == show(get)" failed at t/unit-tests/t-prio-queue.c:43
 #    left: 5
 #   right: 1
 # failed at result[] index 0

That last line repeats "failed" and "result" from the first line.
Shorten it to resemble a similar one in t-ctype and also remove the
incrementation from the first line to avoid possible distractions from
the message of which comparison went wrong where:

 # check "result[j] == show(get)" failed at t/unit-tests/t-prio-queue.c:43
 #    left: 5
 #   right: 1
 #       j: 0

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/unit-tests/t-prio-queue.c

@@ -31,16 +31,18 @@ static void test_prio_queue(int *input, int *result, size_t input_size)
 			get = prio_queue_get(&pq);
 			if (!check(peek == get))
 				return;
-			if(!check_int(result[j++], ==, show(get)))
-				test_msg("failed at result[] index %d", j-1);
+			if (!check_int(result[j], ==, show(get)))
+				test_msg("      j: %d", j);
+			j++;
 			break;
 		case DUMP:
 			while ((peek = prio_queue_peek(&pq))) {
 				get = prio_queue_get(&pq);
 				if (!check(peek == get))
 					return;
-				if(!check_int(result[j++], ==, show(get)))
-					test_msg("failed at result[] index %d", j-1);
+				if (!check_int(result[j], ==, show(get)))
+					test_msg("      j: %d", j);
+				j++;
 			}
 			break;
 		case STACK: