tests: net: rpl: Fix unicast NS sending

If we send unicast NS, then the destination IPv6 address needs
to be in the neighbor cache, otherwise we have a NS sending loop.

Change-Id: I11e8f5d5740248024a0becc77055786cd2e32199
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>