zero: send grep output to /dev/null; set ret=0 at start of 'check repeated...

zero: send grep output to /dev/null; set ret=0 at start of 'check repeated recursive lookups of non recurring TTL=0 responses get new values' test so the failed subtest count is correct

(cherry picked from commit a6507af9)
14 jobs for 569-zero-system-test-failed-to-set-ret-0-and-send-output-to-dev-null-v9_11 in 10 minutes and 5 seconds (queued for 5 seconds)