selftests: Update fib_nexthop_multiprefix to handle missing ping6

Some distributions (e.g., debian buster) do not install ping6. Re-use
the hook in pmtu.sh to detect this and fallback to ping.

Fixes: 735ab2f6

 ("selftests: Add test with multiple prefixes using single nexthop")
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>