Document the purpose of code in Drupal\Core\Test

I think the following classes need to stay in this namespace:

• \Drupal\Core\Test\HttpClientMiddleware\TestHttpClientMiddleware
• \Drupal\Core\Test\StackMiddleware\TestWaitTerminateMiddleware
• \Drupal\Core\Test\PerformanceTestRecorder

\Drupal\Core\Test\ObjectSerialization can probably move to config module.

The others can be moved in 📌 move things from lib/Drupal/Core/Test to core/tests Active and 📌 Create a new component for the test runner Active .