selftests: net: update some helpers to use run_on

selftests: net: update some helpers to use run_on

Update some helpers so that they are capable to run commands on
different targets than the local one. This patch makes the necesasy
modification for those helpers / sections of code which are needed for
the ethtool_rmon.sh test that will be converted in the next patches.

For example, mac_addr_prepare() and mac_addr_restore() used when
STABLE_MAC_ADDRS=yes need to ensure stable MAC addresses on interfaces
located even in other namespaces. In order to do that, append the 'ip
link' commands with a 'run_on $dev' tag.

The same run_on is necessary also when verifying if all the interfaces
listed in NETIFS are indeed available.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://patch.msgid.link/20260330152933.2195885-4-ioana.ciornei@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>