selftests: Add VRF route leaking tests

The objective of the tests is to check that ICMP errors generated while
crossing between VRFs are properly routed back to the source host.

The first ttl test sends a ping with a ttl of 1 from h1 to h2 and parses the
output of the command to check that a ttl expired error is received.

The second ttl test runs traceroute from h1 to h2 and parses the output to
check for a hop on r1.

The mtu test sends a ping with a payload of 1450 from h1 to h2, through
r1 which has an interface with a mtu of 1400 and parses the output of the
command to check that a fragmentation needed error is received.

[ The IPv6 MTU test still fails with the symmetric routing setup. It
  appears to be caused by source address selection picking ::1.  Fixing
  this is beyond the scope of this series. ]

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>