selftests: netfilter: Stabilize rpath.sh

On some systems, neighbor discoveries from ns1 for fec0:42::1 (i.e., the
martian trap address) would happen at the wrong time and cause
false-negative test result.

Problem analysis also discovered that IPv6 martian ping test was broken
in that sent neighbor discoveries, not echo requests were inadvertently
trapped

Avoid the race condition by introducing the neighbors to each other
upfront. Also pin down the firewall rules to matching on echo requests
only.

Fixes: efb056e5 ("netfilter: ip6t_rpfilter: Fix regression with VRF interfaces")
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>