Issue #1938600 by tim.plunkett: Add a FormInterface replacement for confirm_form().

core/lib/Drupal/Core/Form/ConfirmFormBase.php

...s/system/lib/Drupal/system/Tests/Form/ConfirmFormTest.php

core/modules/system/system.module

...ules/system/tests/modules/form_test/form_test.routing.yml

...est/lib/Drupal/form_test/ConfirmFormArrayPathTestForm.php

...es/form_test/lib/Drupal/form_test/ConfirmFormTestForm.php

...views/views_ui/lib/Drupal/views_ui/Form/BreakLockForm.php

...es/views/views_ui/lib/Drupal/views_ui/Form/DeleteForm.php