selftest: netdevsim: Add devlink rate tc-bw test

Test verifies that netdevsim correctly implements devlink ops callbacks
that set tc-bw on leaf or node rate object.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Link: https://patch.msgid.link/20250629142138.361537-4-mbloch@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>