selftests: icmp_redirect: support expected failures

According to a comment in commit 99513cfa ("selftest: Fixes for
icmp_redirect test") the test "IPv6: mtu exception plus redirect" is
expected to fail, because of a bug in the IPv6 logic that hasn't been
fixed yet apparently.

We should probably consider this failure as an "expected failure",
therefore change the script to return XFAIL for that particular test and
also report the total amount of expected failures at the end of the run.

Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>