Establish "core/tests/$type" as a canonical place for test extensions

Problem/Motivation

Currently there is no good place for test modules for core tests.
There are currently modules in core/tests/Drupal/Tests/Core/Extension/modules/, but this causes confusion with nested PSR-4 directories.

For now I want to use this for #3381785: Add kernel tests for hook system and #3379488: Add kernel tests for event system.

Steps to reproduce

n/a

Proposed resolution

Establish core/tests/$type as the canonical place:

• core/tests/modules/ for modules.
• core/tests/themes/ for themes.
• core/tests/profiles/ for profiles?

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet