In order to turn on/off through SNMP DuT under PoE switch, the SNMP key
in some vendors don't directly use the interface number, but a number
shifted a base number.
Define this base number as BM_POE_BASE environment in the runner.
Reviewed-by: Jose Maria Casanova Crespo <jmcasanova@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29306>