selftests: drv-net: Add devlink_rate_tc_bw.py to TEST_PROGS

This makes devlink_rate_tc_bw.py present in the Makefile under the same
directory.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Nimrod Oren <noren@nvidia.com>
Link: https://patch.msgid.link/20251130091938.4109055-2-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>