timing issue with incoming XoT check in `doth` system test

In job #3214134:

I:doth:testing incoming XoT functionality (from the first secondary, StrictTLS via specified IPv6) (5)
I:doth:failed

The check failed because the file ns2/example6.db didn't exist at the time when it's checked (which happens right after client dig AXFR succeeds). The file is present in the artifacts. From the log, it seems that the file was written shortly after the client AXFR has completed.

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

Possible fix: Add retry to the file check.