Lower chance of intermittent serve-stale system test failure

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 however 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.

Closes #3925 (closed)