net: tests: Add tests for IPv4 netmask, gw and subnet compare

Unit tests added for checking network interface netmask, gateway
and IPv4 subnet compare utility functions.

Change-Id: I5b4a07d6a064097ab41ad6552d14181b1631eb53
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>