selftests: mlxsw: router_bridge_lag: Add a new selftest

Add a selftest to verify enslavement to a LAG with upper after fresh
devlink reload.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/373a7754daa4dac32759a45095f47b08a2a869c8.1691498735.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>