package_manager kernel tests are slow

Problem/Motivation

See https://git.drupalcode.org/project/drupal/-/jobs/3373087

Drupal\Tests\package_manager\Kernel\SupportedReleaseValidato 10 passes 190s
Drupal\Tests\package_manager\Kernel\ComposerPluginsValidator 33 passes 388s

Also
https://git.drupalcode.org/project/drupal/-/jobs/3373088
Drupal\Tests\package_manager\Kernel\StageBaseTest 42 passes 271s

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet