tests: let --stress-limit=<N> imply --stress

It does not make much sense that running a test with
--stress-limit=<N> seemingly ignores that option because it does not
stress test at all.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -158,6 +158,7 @@ do
 		esac
 		;;
 	--stress-limit=*)
+		stress=t;
 		stress_limit=${opt#--*=}
 		case "$stress_limit" in
 		*[!0-9]*|0*|"")