Add regress test showing that OpenBSD IPv6 fragment reassembly is

Add regress test showing that OpenBSD IPv6 fragment reassembly is
not affected by FreeBSD-SA-23:06.ipv6 security advisory.  Scapy
test frag6_oversize.py reassembles fragments of a packet too big
to fit.  Test frag6_unfragsize.py also plays games with ECN bits
and hop-by-hop extension header to check overflow protection.  ICMP6
parameter problem responses are expected.  As pf does not generate
such ICMP6 error packets, these tests are only run with frag6_input()
in the IPv6 stack.