Issue #2102653 by tim.plunkett, disasm: Convert test form callbacks to new form controller.

...ules/field/tests/modules/field_test/field_test.entity.inc

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

...les/field/tests/modules/field_test/field_test.routing.yml

...s/field_test/lib/Drupal/field_test/Form/FieldTestForm.php

...les/file/tests/file_module_test/file_module_test.info.yml

...dules/file/tests/file_module_test/file_module_test.module

.../file/tests/file_module_test/file_module_test.routing.yml

...t/lib/Drupal/file_module_test/Form/FileModuleTestForm.php

...ests/language_elements_test/language_elements_test.module

...language_elements_test/language_elements_test.routing.yml

.../language_elements_test/Form/LanguageElementsTestForm.php

...b/Drupal/search_embedded_form/Form/SearchEmbeddedForm.php

.../modules/search_embedded_form/search_embedded_form.module

...les/search_embedded_form/search_embedded_form.routing.yml

...s/system/lib/Drupal/system/Tests/Batch/ProcessingTest.php

...system/lib/Drupal/system/Tests/Form/FileInclusionTest.php

...stem/tests/modules/ajax_forms_test/ajax_forms_test.module

...tests/modules/ajax_forms_test/ajax_forms_test.routing.yml

...est/lib/Drupal/ajax_forms_test/Form/AjaxFormsTestForm.php

...modules/system/tests/modules/batch_test/batch_test.module