Mark slow kernel tests with @group slow

Problem/Motivation

Kernel tests are the highest CPU users in gitlab jobs.

If we mark slow tests, we can ensure that the end of the job is running lots of tests, not one test, and that may allow the same number of tests to complete in the same amount of time, but with lower concurrency.

If we can lower the concurrency, we can potentially also reduce the CPU request, without having to add an extra job.

Even if we add an extra job in the end, this may allow jobs to finish quicker and free up runners.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet