test: use /run/ for temporary files in TEST-07-PID1.issue-14566

And delete the file before starting, to be safe against reruns

Follow-up for c1566ef0d2

test/integration-tests/TEST-07-PID1/TEST-07-PID1.units/issue14566-repro.sh

@@ -2,6 +2,6 @@
 # SPDX-License-Identifier: LGPL-2.1-or-later
 
 sleep infinity &
-echo $! >/leakedtestpid
+echo $! >/run/leakedtestpid
 systemd-notify --ready
 wait $!

test/units/TEST-07-PID1.issue-14566.sh

@@ -13,10 +13,11 @@ if [[ -v ASAN_OPTIONS ]]; then
     exit 0
 fi
 
+rm -f /run/leakedtestpid
 systemctl start issue14566-repro
 systemctl status issue14566-repro
 
-leaked_pid=$(cat /leakedtestpid)
+leaked_pid=$(cat /run/leakedtestpid)
 
 systemctl stop issue14566-repro
 timeout 30 bash -c 'while systemctl is-active issue14566-repro; do sleep .5; done'