selftests: net: bridge: add vlan mcast_last_member_count/interval tests

Add tests which verify the default values of mcast_last_member_count
mcast_last_member_count and also try to change them.

TEST: Vlan mcast_last_member_count global option default value      [ OK ]
TEST: Vlan mcast_last_member_interval global option default value   [ OK ]
TEST: Vlan 10 mcast_last_member_count option changed to 3           [ OK ]
TEST: Vlan 10 mcast_last_member_interval option changed to 200      [ OK ]

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