test: spawn the to-be-killed-on-soft-reboot units with --collect

test: spawn the to-be-killed-on-soft-reboot units with --collect

Otherwise they might leave stuff behind if they don't respond fast
enough to the first SIGTERM and get SIGKILLEd, which then breaks reusing
the unit name further in the test:

[ 2993.620849] H testsuite-82.sh[43]: + systemd-run -p Type=exec -p DefaultDependencies=no -p IgnoreOnIsolate=yes --unit=testsuite-82-nosurvive.service sleep infinity
[ 2993.628686] H systemd[1]: testsuite-82-nosurvive.service: About to execute: /usr/bin/sleep infinity
[ 2993.628886] H systemd[1]: testsuite-82-nosurvive.service: Forked /usr/bin/sleep as 65
[ 2993.629328] H systemd[1]: testsuite-82-nosurvive.service: Changed dead -> start
...
[ 2993.699892] H testsuite-82.sh[43]: + systemctl --no-block --check-inhibitors=yes soft-reboot
[ 2993.704326] H systemd-logind[41]: The system will soft-reboot now!
...
[ 3001.249302] H systemd[1]: Sending SIGKILL to PID 65 (sleep).
...
[ 3001.303158] H testsuite-82.sh[136]: + systemd-notify '--status=Second Boot'
...
[ 3001.409504] H testsuite-82.sh[136]: + systemd-run -p Type=exec --unit=testsuite-82-nosurvive.service sleep infinity
[ 3001.414061] H testsuite-82.sh[165]: Failed to start transient service unit: Unit testsuite-82-nosurvive.service was already loaded or has a fragment file.

Spotted in Ubuntu CI.