Test case added for IPv6 neighbors. This will add more than
CONFIG_NET_IPV6_MAX_NEIGHBORS neighbors. Network stack should
remove oldest neighbor which is in STALE state and it should
add new neighbor. So call to net_ipv6_nbr_add() should succeed.
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>