lib/match: Add mask bits for nd_target for ICMPv6

nd_target field needs to be included in the wildcards masks for it to be
processed properly.

Add a testcase to test IIPv6 Neighbor Discovery parsing and processing.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>