system.module is included in our PHPUnit tests

Problem/Motivation

system.module is included by a phpunit which means that you can have possible unexpected side effects. For example, #2349991: Provide a trait for categorizing plugin managers and use it for conditions and actions adds a unit test that creates the system_get_info() so it can unit test but then you get the following error if you run all the phpunit tests.

PHP Fatal error:  Cannot redeclare system_get_info() (previously declared in /Volumes/devdisk/dev/drupal/core/tests/Drupal/Tests/Core/Plugin/CategorizingPluginManagerTraitTest.php:150) in /Volumes/devdisk/dev/drupal/core/modules/system/system.module on line 852

Fatal error: Cannot redeclare system_get_info() (previously declared in /Volumes/devdisk/dev/drupal/core/tests/Drupal/Tests/Core/Plugin/CategorizingPluginManagerTraitTest.php:150) in /Volumes/devdisk/dev/drupal/core/modules/system/system.module on line 852

Proposed resolution

Fix core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest

Remaining tasks

Review

User interface changes

None

API changes

None