Actually create the tap(4) with a negative device number. Doh.

tests/sbin/ifconfig/t_tap.sh

@@ -1,4 +1,4 @@
-# $NetBSD: t_tap.sh,v 1.2 2020/06/25 15:43:26 jruoho Exp $
+# $NetBSD: t_tap.sh,v 1.3 2020/06/25 16:52:49 jruoho Exp $
 #
 # Copyright (c) 2020 The NetBSD Foundation, Inc.
 # All rights reserved.
@@ -88,7 +88,7 @@ overflow_body() {
 
 overflow_cleanup() {
 
-	ifconfig tap65537
+	ifconfig tap65537 create
 
 	if [ $? -eq 0 ]; then
 		ifconfig tap65537 destroy