tests: net: ipv6: Increase the multicast address count
tests: net: ipv6: Increase the multicast address count

Increasing the IPv6 multicast address count from 2 to 5 so that
tests will always run ok.

Fixes #4401

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

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