Ensure tests don't run twice

Problem/Motivation

Reported by @berdir in slack.

=Drupal\KernelTests\Core\Plugin\DefaultPluginManagerTest shows up in both kernel test group 3/4 and 4/4

https://git.drupalcode.org/issue/drupal-3265945/-/jobs/3370130

https://git.drupalcode.org/issue/drupal-3265945/-/jobs/3370131https://gi...

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet