[Tests] Fix check for running process to prevent setup failures

If a test causes a server to crash, a pid file with the pid of
the crashed server will be left behind. The teardown phase would
then fail due to a non-zero return of the ps command when assigning
the pid to $PID. This changes the test to skip killing a process
that has already exited without failing.

tests/XRootD/test.sh

@@ -147,7 +147,7 @@ function teardown() {
 	# Kill all processes that created pid files and are still running
 	for PIDFILE in *.pid; do
 		test -s "${PIDFILE}" || continue
-		PID="$(ps -o pid= "$(cat "${PIDFILE}")")"
+		PID="$(ps -o pid= "$(cat "${PIDFILE}")" || true)"
 		if test -n "${PID}"; then
 			kill -s TERM "${PID}"
 		fi