test: fix flaky TEST-07-PID1.socket-defer.sh

test: fix flaky TEST-07-PID1.socket-defer.sh

The socket's SubState transitions from 'running' to 'listening' shortly
after the triggered service becomes inactive, so the assert can race and
observe the stale 'running' state:

[ 1882.425335] systemd[1]: TEST-07-PID1-socket-defer-23279.service: Changed dead -> running
[ 1882.495150] TEST-07-PID1.sh[20535]: ++ systemctl show TEST-07-PID1-socket-defer-23279.socket -P SubState
[ 1882.514239] TEST-07-PID1.sh[20509]: + assert_eq running listening
[ 1882.510529] systemd[1]: TEST-07-PID1-socket-defer-23279.socket: Flushing socket before listening.
[ 1882.510559] systemd[1]: TEST-07-PID1-socket-defer-23279.socket: Changed running -> listening

Poll for 30s instead of directly asserting to try and make it more robust