unit tests failure with mysql 5.7.39 and freebsd 12.

unit test failures:

• https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/lastCompletedBuild/testReport/(root)/MySqlConfigBackendDHCPv6Test/run_tests___freebsd_12_1_amd64___freebsd_12_1_amd64_results___globalOptions6WithServerTagsTest/
• https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/lastCompletedBuild/testReport/(root)/MySqlConfigBackendDHCPv6Test/run_tests___freebsd_12_1_amd64___freebsd_12_1_amd64_results___globalOptions6WithServerTagsTest/
• https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/lastCompletedBuild/testReport/(root)/MySqlConfigBackendDHCPv6Test/run_tests___freebsd_12_1_amd64___freebsd_12_1_amd64_results___unassignedSubnet6Test/

19:27:28  MySQL:
19:27:28    MYSQL_VERSION:   5.7.39
19:27:28    MYSQL_CPPFLAGS:  -I/usr/local/include/mysql 
19:27:28    MYSQL_LIBS:      -L/usr/local/lib/mysql -lmysqlclient -lpthread -lm -lrt -lexecinfo -lssl -lcrypto -lssl -lcrypto

test report: https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/957/testReport/

Most of systems we are using at the build farm are using mysql version 8 or higher. Except centos 7 (5.5.68) and ubuntu 18.04 (5.7.39) but tests are not failing there.