tests: disable interactive hunk selection tests if perl is not available

These are all backed by git-add--interactive.perl under the hood.

Signed-off-by: Jeff King <peff@peff.net>
Acked-By: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-patch-mode.sh

@@ -1,5 +1,10 @@
 . ./test-lib.sh
 
+if ! test_have_prereq PERL; then
+	say 'skipping --patch tests, perl not available'
+	test_done
+fi
+
 set_state () {
 	echo "$3" > "$1" &&
 	git add "$1" &&