Update serve-stale system test

Update serve-stale system test

The serve-stale system test was intermittently failing due to a timing
issue:

    I:serve-stale:check stale data.example TXT was refreshed...
    I:serve-stale:failed

The RRset is refreshed, however, it first checks for an expected log
line, prior checking that the stale data.example TXT was refreshed
(using dig). This log line is there to ensure the record is actually
refreshed before we start querying again. Alternatively we could just
retry_quiet 10 <wait for dig output matches expectations>. It would
lower the chances for intermittent test failures, since there is no
longer a "check for log line, sleep one second if check fails, check
for log line, ...", prior to the check.