When calling "rndc dnssec -checkds", it may take some milliseconds
before the appropriate changes have been written to the state file.
Add retry_quiet mechanisms to allow the write operation to finish.
Also retry_quiet the check for the next key event. A "rndc dnssec"
command may trigger a zone_rekey event and this will write out
a new "next key event" log line, but it may take a bit longer than
than expected in the tests.
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