selftests: drv-net: introduce Iperf3Runner for measurement use cases

selftests: drv-net: introduce Iperf3Runner for measurement use cases

GenerateTraffic was added to spin up long-running iperf3 load, mainly
to drive high PPS background traffic. It was never meant to provide
stable throughput numbers, and trying to repurpose it for measurement
does not make sense.

Introduce Iperf3Runner to allow tests to split out server/client
configuration, control start/stop, and collect JSON output for
analysis. This makes it possible to measure bandwidth directly when
validating egress shaping.

GenerateTraffic stays as the background load generator, reusing the
common iperf3 helpers under the hood.

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/20251130091938.4109055-3-cjubran@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>