Wait until "rndc dumpdb" completes
"rndc dumpdb" works asynchronously, i.e. the requested dump may not yet be fully written to disk by the time "rndc" returns. Prevent false positives for the "serve-stale" system test by only checking dump contents after the line indicating that it is complete is written.
!1712 with michal/serve-stale-system-test-wait-for-dump-completion in 18 minutes and 10 seconds (queued for 11 seconds)41 jobs for