dummynet tests: disable for now

Disable the dummynet tests when running the ci tests. This avoids
running into the panic described in https://reviews.freebsd.org/D33064
(where an interface is removed but a dummynet queued packet still has a
pointer to it).

These tests can be re-enabled when the work in
https://reviews.freebsd.org/D33267 lands.

Sponsored by:	Rubicon Communications, LLC ("Netgate")

tests/sys/netpfil/common/utils.subr

@@ -111,6 +111,10 @@ dummynet_init()
 		atf_skip "This test requires dummynet"
 	fi
 
+	if [ "$(atf_config_get ci false)" = "true" ]; then
+		atf_skip "Skip to avoid dummynet_send() panic. See https://reviews.freebsd.org/D33064"
+	fi
+
 	case $firewall in
 	ipfw|pf)
 		# Nothing. This is okay.