selftests: test_vxlan_fdb_changelink: Add a test for MC remote change

Changes to MC remote need to be reflected in actual group memberships.
Add a test to verify that it is the case.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>