tests: net: ipv6: Make ICMP handler struct static

Make sure that we will never access ICMP handler struct
that is allocated from stack.
Without this change, there was mysterious crashes if using
native_sim board where the handler function was pointing to
stack data which contained garbage when the test was run.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>