perf-test: propagate non-zero returns from /usr/bin/time

Unlike in the correctness tests, the most common cause of non-zero
return seems to be the user interrupting, so killing the run seems
like the friendly thing to do.

performance-test/perf-test-lib.sh

@@ -51,7 +51,9 @@ time_run () {
     if test "$verbose" != "t"; then exec 4>test.output 3>&4; fi
     if ! eval >&3 "/usr/bin/time -f '%e\t%U\t%S\t%M\t%I\t%O' $2" ; then
 	test_failure=$(($test_failure + 1))
+	return 1
     fi
+    return 0
 }
 
 time_done () {