Symfony version(s) affected: 3.4, 4.4, 5.1, 5.2

Description
See sebastianbergmann/phpunit#4297.

The Symfony testsuite makes heavy use of the at() matcher to model multiple expected calls to a test double. The PHPUnit project plans to remove that functionality with PHPUnit 10.

How to reproduce
Run the testsuite with PHPUnit 9.3.

Possible Solution

• @greg0ire has worked on a PR for Doctrine Annotations: Remove calls to TestCase::at() doctrine/annotations#345. Maybe we can apply similar changes.
• Work with the PHPUnit project on a proper replacement for that feature.
• Switch to a different mocking framework. 🙈

Additional context
When running the 3.4 testsuite with PHPUnit 9.3, about 170 deprecation warnings are logged regarding the planned removal of that functionality: https://travis-ci.org/github/symfony/symfony/jobs/716119378

Related to #37564