selftests: forwarding: extend ethtool_std_stats_get with pause statistics

selftests: forwarding: extend ethtool_std_stats_get with pause statistics

Even though pause frame statistics are not exported through the same
ethtool command, there is no point in adding another helper just for
them. Extent the ethtool_std_stats_get() function so that we are able to
interrogate using the same helper all the standard statistics.

And since we are touching the function, convert the initial ethtool call
as well to the jq --arg form in order to be easier to read.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260330152933.2195885-2-ioana.ciornei@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>