Retry multiple times in doth test when checking if the zone file exists

Retry multiple times in doth test when checking if the zone file exists

Dumping of the freshly transferred zone file can take some time.

Retry 5 times before failing.

The log excerpt below shows such a case, when dumping lasted more than
two seconds.

    06-Mar-2023 09:32:09.973 zone example6/IN: Transfer started.
    06-Mar-2023 09:32:10.301 zone example6/IN: zone transfer finished: success
    06-Mar-2023 09:32:10.301 zone_dump: zone example6/IN: enter
    06-Mar-2023 09:32:11.789 client @0x7fe9ab435d68 10.53.0.10#44113 (example6): AXFR request
    06-Mar-2023 09:32:11.801 client @0x7fe9ab435d68 10.53.0.10#44113 (example6): transfer of 'example6/IN': AXFR ended: 5 messages, 2676 records, 55815 bytes, 0.011 secs (5074090 bytes/sec) (serial 1397051952)
    06-Mar-2023 09:32:12.409 zone_gotwritehandle: zone example6/IN: enter
    06-Mar-2023 09:32:12.421 dump_done: zone example6/IN: enter
    06-Mar-2023 09:32:12.421 zone_journal_compact: zone example6/IN: target journal size 53044

(cherry picked from commit 5d5d4b523be029b4781fc53c3c95143d783db39f)