Issue #2252991 by neclimdul: PhpUnit tests for ModuleHandler.

core/lib/Drupal/Core/Extension/ModuleHandler.php

core/tests/Drupal/Tests/Core/Extension/ModuleHandlerTest.php

...re/Extension/modules/module_handler_test/hook_include.inc

.../modules/module_handler_test/module_handler_test.info.yml

...on/modules/module_handler_test/module_handler_test.module

...ule_handler_test_added/module_handler_test_added.hook.inc

...ule_handler_test_added/module_handler_test_added.info.yml

...odule_handler_test_added/module_handler_test_added.module

...odule_handler_test_all1/module_handler_test_all1.info.yml

.../module_handler_test_all1/module_handler_test_all1.module

...odule_handler_test_all2/module_handler_test_all2.info.yml

.../module_handler_test_all2/module_handler_test_all2.module

core/tests/bootstrap.php