Mark more KernelTests with @group slow

Doesn't affect the random failure so no point splitting this.

Actually there's a good reason to do this.

In 📌 Investigate realistic resource requests for core jobs Active we want to reduce the resources used for test runners to a minimum reasonable point.

If we compare https://git.drupalcode.org/project/drupal/-/jobs/143679 (Test run duration: 4 min) and https://git.drupalcode.org/project/drupal/-/jobs/143680 (Test run duration: 3 min 46 sec) they are not dramatically different, but if we constrain resources it's possible they'll diverge more.

If we tag the slow tests, it'll equally distribute the longest running tests between the two jobs and balance them more, hopefully.