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 <[email protected]>
Acked-By: Thomas Rast <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: peff

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" &&