Currently, ns3 in the "mirror" system test sends trust anchor telemetry
queries every second as it is started with "-T tat=1". Given the number
of trust anchors configured on ns3 (9), TAT-related traffic clutters up
log files, hindering troubleshooting efforts. Increase TAT query
interval to 3 seconds in order to alleviate the issue.
Note that the interval chosen cannot be much higher if intermittent test
failures are to be avoided: TAT queries are only sent after the
configured number of seconds passes since resolver startup. Quick
experiments show that even on contemporary hardware, ns3 should be
running for at least 5 seconds before it is first shut down, so a
3-second TAT query interval seems to be a reasonable, future-proof
compromise. Ensure the relevant check is performed before ns3 is first
shut down to emphasize this trade-off and make it more clear by what
time TAT queries are expected to be sent.
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