selftests: Add l2tp tests

Add IPv4 and IPv6 l2tp tests. Current set is over IP and with
IPsec.

v2
- add l2tp.sh to TEST_PROGS in Makefile

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>