ipv6 tests

Add a simple ping6 test as well.

MFC after:	2 weeks
Sponsored by:	Netflix

tests/sys/netinet6/exthdr.sh

@@ -74,6 +74,8 @@ exthdr_body() {
 	pyname=$(atf_get ident)
 	pyname=${pyname%*_[0-9]}
 
+	atf_check -o ignore -s exit:0 ping6 -c 3 -q -o ${ip6b}
+
 	atf_check -s exit:0 $(atf_get_srcdir)/${pyname}.py \
 		--sendif ${epair}a --recvif ${epair}a \
 		--src ${ip6a} --to  ${ip6b}