testing: Fix route-based/net2net-xfrmi-ike scenario

testing: Fix route-based/net2net-xfrmi-ike scenario

On newer systems, the upper hard limit for open file descriptors (see
`ulimit -H -n`) was increased from 4096 to 524288.  Due to how python-daemon
closes potentially open file descriptors (basically stores them in a set,
removes those excluded by config, and loops through all of them), the updown
script was either killed immediately (by the OOM killer) or not ready yet
when updown events occurred.