selftests: drv-net: Use Iperf3Runner in devlink_rate_tc_bw.py

Replace the inline iperf3 subprocess and JSON parsing with
Iperf3Runner.

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/20251130091938.4109055-4-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
1 file changed