Move all forum tests to the module in preparation of removal in Drupal 10

Problem/Motivation

Some test for forum module are scattered around in other modules. We need to consolidate these tests into the modules directory (core/module/forum) and into the modules namespace.

This way it will be easier to move the current Core module almost as-is to the Contrib realm, and make it easier to fix possible security issues from the contrib in 9.4 later.

Steps to reproduce

Proposed resolution

Move all tests to the forum module.

Remaining tasks

Move all tests to forum module.

User interface changes

API changes

Data model changes

Release notes snippet