TEST-15: add daemon-reload in one place

TEST-15: add daemon-reload in one place

Quoting https://github.com/systemd/systemd/pull/25050#discussion_r998721845:

This part seems to be quite racy, at least in the C8S job:

[ 1767.520856] H testsuite-15.sh[35]: *** test transient slice drop-ins
[ 1767.520856] H testsuite-15.sh[35]: + mkdir -p /etc/systemd/system/slice.d
[ 1767.522480] H testsuite-15.sh[35]: + mkdir -p /etc/systemd/system/a-.slice.d
[ 1767.524992] H testsuite-15.sh[35]: + mkdir -p /etc/systemd/system/a-b-.slice.d
[ 1767.526799] H testsuite-15.sh[35]: + mkdir -p /etc/systemd/system/a-b-c.slice.d
[ 1767.528302] H testsuite-15.sh[35]: + echo -e '[Unit]\nDocumentation=man:drop1'
[ 1767.528434] H testsuite-15.sh[35]: + echo -e '[Unit]\nDocumentation=man:drop2'
[ 1767.528519] H testsuite-15.sh[35]: + echo -e '[Unit]\nDocumentation=man:drop3'
[ 1767.528595] H testsuite-15.sh[35]: + echo -e '[Unit]\nDocumentation=man:drop4'
[ 1767.528676] H testsuite-15.sh[35]: + systemctl cat a-b-c.slice
[ 1767.541321] H systemctl[1042]: No files found for a-b-c.slice.
[ 1767.542854] H systemd[1]: testsuite-15.service: Main process exited, code=exited, status=1/FAILURE
[ 1767.542995] H systemd[1]: testsuite-15.service: Failed with result 'exit-code'.
[ 1767.543360] H systemd[1]: Failed to start testsuite-15.service.
[ 1767.543542] H systemd[1]: testsuite-15.service: Consumed 1.586s CPU time.
[ 1767.543938] H systemd[1]: Reached target testsuite.target.
[ 1767.545737] H systemd[1]: Starting end.service...