test: use KILL instead of SIGKILL in TEST-52-HONORFIRSTSHUTDOWN

test: use KILL instead of SIGKILL in TEST-52-HONORFIRSTSHUTDOWN

SIG-prefixed signals for `kill` are not POSIX compliant, so on Ubuntu CI
(which defaults to dash instead of bash) the TEST-52 contains following
error:

[ 9693.549638] sh[51]: + systemctl poweroff --no-block
[ 9693.553130] systemd-logind[26]: System is powering down.
[ 9693.608911] sh[54]: /bin/sh: 1: kill: Illegal option -S

This can be reproduced manually as well, either by running dash, or bash
in POSIX mode:

$ dash -c 'kill -SIGKILL 123'
dash: 1: kill: Illegal option -S

$ bash --posix -c 'kill -SIGKILL 123'
bash: line 0: kill: SIGKILL: invalid signal specification