TestCase::getMockClass() is deprecated in PHPUnit 9.6 and removed from PHPUnit 10

Problem/Motivation

TestCase::getMockClass() is removed from PHPUnit 10.

In PHPUnit 9.6, using getMockClass() will yield a deprecation error,

Therefore the use of the method needs to be replaced.

See https://github.com/sebastianbergmann/phpunit/issues/5064

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet