tests: net: ipv6: Calculate ICMPv6 checksum for RA message
tests: net: ipv6: Calculate ICMPv6 checksum for RA message

The IPv6 test was failing because the ICMPv6 checksum was
wrong.

Signed-off-by: Jukka Rissanen <f9f077d1da4c7aa947f9864fa5a28c3588f6a45d@linux.intel.com>

explora26/zephyr,ldts/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr