selftests: pmtu: Introduce list_flush_ipv4_exception test case

This test checks that route exceptions can be successfully listed and
flushed using ip -6 route {list,flush} cache.

v7: No changes

v6:
  - Merge this patch into series including fix, as it's also targeted
    for net-next
  - Drop left-over print of 'ip route list cache | wc -l'

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>