Try to rebalance kernel tests between gitlab runners (@group #slow again)

Problem/Motivation

Functional test jobs are nearly consistently under four minutes, which means kernel tests are now running longer. The jobs were more even than this previously, but as tests are added things move around, and it's as much about the distribution of tests as any individual test.

Before:
https://git.drupalcode.org/project/drupal/-/jobs/2177702 4m9s
https://git.drupalcode.org/project/drupal/-/jobs/2177703 5m13s
https://git.drupalcode.org/project/drupal/-/jobs/2177704 2m23s

After:
https://git.drupalcode.org/project/drupal/-/jobs/2193124 3 min 18 sec
https://git.drupalcode.org/project/drupal/-/jobs/2193125 2 min 55 sec
https://git.drupalcode.org/project/drupal/-/jobs/2193126 2 min 18 sec

Generally precision isn't massively important here - we can just add @group #slow to a few tests with 20+ test methods, and those generally take 20-30 seconds to run instead of 1-3 seconds for a single method test. The group slow logic will then distribute those tests equally among the three runners, and hopefully we'll end up with three jobs at least than four minutes each.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet