HAServiceTest.sendSuccessfulUpdatesAuthorizedMultiThreading sometimes fails

This time it happened on distcheck on CentOS 8.

https://jenkins.aws.isc.org/job/kea-dev/job/distcheck/415/execution/node/136/log/?consoleFull

16:04:40  [ RUN      ] HAServiceTest.sendSuccessfulUpdatesAuthorizedMultiThreading
16:04:40  ../../../../../../../src/hooks/dhcp/high_availability/tests/ha_service_unittest.cc:1096: Failure
16:04:40  Expected equality of these values:
16:04:40    2
16:04:40    factory3_->getResponseCreator()->getReceivedRequests().size()
16:04:40      Which is: 1
16:04:40  ../../../../../../../src/hooks/dhcp/high_availability/tests/ha_service_unittest.cc:1102: Failure
16:04:40  Value of: update_request3
16:04:40    Actual: false
16:04:40  Expected: true
16:04:40  [  FAILED  ] HAServiceTest.sendSuccessfulUpdatesAuthorizedMultiThreading (2 ms)