Add @slow group to newly split kernel jobs

Problem/Motivation

In 📌 [PP-1] Reevaluate the break-up of the various testsuites on GitLab Postponed kernel jobs were split into 3

comment from @catch

This successfully lowers the long runtime of the two kernel jobs, but it does it a bit unevenly, so we may have a few more @group #slow to add to even things out between the three jobs.

Steps to reproduce

NA

Proposed resolution

Add @group slow to to even out kernel tests.

Remaining tasks

Implement
Review

User interface changes

NA

API changes

NA

Data model changes

NA

Release notes snippet

NA