test: slay must cache oldpid to an instance specific ramdisk

Running tests in massive parallel causes failing startk-kill-service.sh.
This in turn was caused by the shared test tool 'slay' that cached its
"oldpid" file to a shared area of the rootfs (tenv-root).  It must use a
test instance-specific ramdisk.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

test/skel/bin/slay

@@ -48,7 +48,7 @@ while [ "$pid" -le 1 ]; do
 	break
 done
 
-echo "$pid" > oldpid
+echo "$pid" > /tmp/oldpid
 
 #echo "PID $pid, kill -9 ..."
 kill -9 "$pid"