PT-2295 - fix tests on distributions

[svetasmirnova]

Fixed test failures for PS MYSQL 8.0. Test results are now green.

All distributions: pt-unit-tests-all, build 24 Re-run test for buster: pt-unit-tests, build 179

• [ ] The contributed code is licensed under GPL v2.0
• [ ] Contributor Licence Agreement (CLA) is signed
• [ ] util/update-modules has been ran
• [ ] Documentation updated
• [ ] Test suite update

[render[bot]]

Your Render PR Server URL is https://percona-toolkit-pr-769.onrender.com.

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