Issue #2715741 by Mile23, pfrenssen: Fix 'Drupal.Classes.FullyQualifiedNamespace' coding standard

core/includes/common.inc

core/includes/form.inc

core/lib/Drupal/Core/DrupalKernel.php

core/lib/Drupal/Core/PageCache/DefaultRequestPolicy.php

core/modules/aggregator/src/Tests/ItemCacheTagsTest.php

...odules/big_pipe/src/Tests/BigPipePlaceholderTestCases.php

core/modules/big_pipe/src/Tests/BigPipeTest.php

...tests/modules/big_pipe_test/src/BigPipeTestController.php

...tests/src/Unit/Render/Placeholder/BigPipeStrategyTest.php

core/modules/comment/src/Tests/CommentCacheTagsTest.php

...all_dependency_test/config_install_dependency_test.module

.../modules/contact_storage_test/contact_storage_test.module

core/modules/field/field.module

...pdate/drupal-8.views_entity_reference_plugins-2429191.php

.../modules/field/tests/modules/field_test/field_test.module

...ules/field_third_party_test/field_third_party_test.module

core/modules/filter/tests/src/Kernel/FilterAPITest.php

core/modules/forum/forum.module

core/modules/locale/locale.bulk.inc

...es/locale/tests/src/Kernel/LocaleConfigSubscriberTest.php