t-prio-queue: shorten array index message

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>