Move TestSuite/* into a dedicated namespace

Problem/Motivation

i'm a bit confused about the namespace overlap of \Drupal\Tests\TestSuites\TestSuiteBase and \Drupal\Tests\TestSuites\TestSuiteBaseTest

core/tests/TestSuites/TestSuiteBase.php
and core/tests/Drupal/Tests/TestSuites/TestSuiteBaseTest.php have the same namespace, but are living in total different folders.

On top of that \Drupal\Tests\TestSuites\TestSuiteBaseTest relies on core/tests/TestSuites/TestSuiteBase.php being loaded before.

Proposed resolution

• Move the TestSuite files from \Drupal\Tests\TestSuites to \Drupal\TestSuites
• Add a manual require_once into \Drupal\Tests\TestSuites\TestSuiteBaseTest

Remaining tasks

User interface changes

API changes

Data model changes