test: let kernel OOM-kill a child process instead of the main one

test: let kernel OOM-kill a child process instead of the main one

This test occasionally fails due to a race where systemd processes
kernel's SIGKILL before the OOM notification, so the test service dies
with Result=signal instead of the expected Result=oom-kill:

[   51.008765] TEST-55-OOMD.sh[906]: + systemd-run --wait --unit oom-kill -p OOMPolicy=kill -p Delegate=yes -p DelegateSubgroup=init.scope /tmp/script.sh
[   51.048747] TEST-55-OOMD.sh[907]: Running as unit: oom-kill.service; invocation ID: 456645347d554ea2878463404b181bd8
[   51.066296] sysrq: Manual OOM execution
[   51.066596] kworker/1:0 invoked oom-killer: gfp_mask=0xcc0(GFP_KERNEL), order=-1, oom_score_adj=0
[   51.066915] CPU: 1 UID: 0 PID: 27 Comm: kworker/1:0 Not tainted 6.17.1-arch1-1 #1 PREEMPT(full)  d2b229857b2eb4001337041f41d3c4f131433540
[   51.066919] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Arch Linux 1.17.0-2-2 04/01/2014
[   51.066921] Workqueue: events moom_callback
[   51.066928] Call Trace:
[   51.066931]  <TASK>
[   51.066936]  dump_stack_lvl+0x5d/0x80
[   51.066942]  dump_header+0x43/0x1aa
<...snip...>
[   51.087814] 47583 pages reserved
[   51.087969] 0 pages cma reserved
[   51.088208] 0 pages hwpoisoned
[   51.088519] Out of memory: Killed process 908 (sleep) total-vm:3264kB, anon-rss:256kB, file-rss:1916kB, shmem-rss:0kB, UID:0 pgtables:44kB oom_score_adj:1000
[   51.090263] TEST-55-OOMD.sh[907]:           Finished with result: signal
[   51.094416] TEST-55-OOMD.sh[907]: Main processes terminated with: code=killed, status=9/KILL
[   51.094898] TEST-55-OOMD.sh[907]:                Service runtime: 58ms
[   51.095436] TEST-55-OOMD.sh[907]:              CPU time consumed: 22ms
[   51.095854] TEST-55-OOMD.sh[907]:                    Memory peak: 1.6M (swap: 0B)
[   51.096722] TEST-55-OOMD.sh[912]: ++ systemctl show oom-kill -P Result
[   51.106549] TEST-55-OOMD.sh[879]: + assert_eq signal oom-kill
[   51.107394] TEST-55-OOMD.sh[913]: + set +ex
[   51.108256] TEST-55-OOMD.sh[913]: FAIL: expected: 'oom-kill' actual: 'signal'
[FAILED] Failed to start TEST-55-OOMD.service.

To mitigate this, let's spawn a child process and move it to the
subcgroup to get killed instead of the main process, so systemd has more
time to react to the OOM notification and terminate the service with the
expected oom-kill result.