Race in 'clear signing records' in dnssec system test.

Job #892140 failed for 3353bbbe:

Updates to the zone are task locked and rndc command to clear the key returns as it is being processed. The subsequent rndc command to check can happen before the zone has updated.

I:dnssec:clear signing records (170)
I:dnssec:ns3 Done signing with key 14616/NSEC3RSASHA1
I:dnssec:failed

echo_i "clear signing records ($n)"
{ rndccmd 10.53.0.3 signing -clear all update-nsec3.example > /dev/null; } 2>&1 || ret=1
sleep 1
{ rndccmd 10.53.0.3 signing -list update-nsec3.example > signing.out; } 2>&1
grep -q "No signing records found" signing.out || {
        ret=1
        sed 's/^/ns3 /' signing.out | cat_i
}
n=$((n+1))
test "$ret" -eq 0 || echo_i "failed"
status=$((status+ret))

18-May-2020 22:26:03.628 received control channel command 'signing -clear all update-nsec3.example'
18-May-2020 22:26:03.628 keydone: zone update-nsec3.example/IN: enter
...
18-May-2020 22:26:03.640 del update-nsec3.example.      0       IN      TYPE65534 \# 5 0739180001
...
18-May-2020 22:26:04.708 received control channel command 'signing -list update-nsec3.example'
...
18-May-2020 22:26:05.032 zone_needdump: zone update-nsec3.example/IN: enter