Occasional false negatives for this test observed on Linux machines
(CentOS 7 [1], Fedora 25 [2]) are caused by a known kernel bug described
in KB AA-01213 [3].

[1] https://jenkins.isc.org/view/BIND/job/bind9-master-centos7-amd64-1/311/testReport/bind/system/nsupdate/
    https://jenkins.isc.org/view/BIND/job/bind9-master-centos7-amd64-1/355/testReport/bind/system/nsupdate/
    https://jenkins.isc.org/view/BIND/job/bind9-master-centos7-amd64-1/357/testReport/bind/system/nsupdate/

[2] https://jenkins.isc.org/view/BIND/job/bind9-master-fedora25-amd64-1/329/testReport/bind/system/nsupdate/
    https://jenkins.isc.org/view/BIND/job/bind9-master-fedora25-amd64-1/332/testReport/bind/system/nsupdate/
    https://jenkins.isc.org/view/BIND/job/bind9-master-fedora25-amd64-1/344/testReport/bind/system/nsupdate/

[3] https://kb.isc.org/article/AA-01213