Fix 055.backend_all_down test failure.

Fix 055.backend_all_down test failure.

The test fails because pgpool zombie child process remains. Actually
the failover process is properly performed but when the shutdown
script is executed in background, output to stdout/stderr was blocked,
and this could cause the zombie process syndrome. Soltution is,
redirecting stdout/stderr to /dev/null when sponing the shutdown
script in background.