selftests: net: bridge: add test for mldv2 inc -> allow report

The test checks for the following case:
   Router State  Report Received  New Router State     Actions
   INCLUDE (A)     ALLOW (B)      INCLUDE (A+B)        (B)=MALI

Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>