Issue #3495499 by nicxvan, nikolay shapovalov: Move helpers in file_test.module and delete it

core/.phpstan-baseline.php

core/modules/file/tests/file_test/file_test.module

core/modules/file/tests/file_test/src/FileTestCdn.php

core/modules/file/tests/file_test/src/FileTestHelper.php

core/modules/file/tests/file_test/src/Hook/FileTestHooks.php

...test/src/EventSubscriber/FileValidationTestSubscriber.php

core/modules/file/tests/src/Functional/DownloadTest.php

...modules/file/tests/src/Functional/FileManagedTestBase.php

.../modules/file/tests/src/Functional/SaveUploadFormTest.php

core/modules/file/tests/src/Functional/SaveUploadTest.php

core/modules/file/tests/src/Kernel/DeleteTest.php

...modules/file/tests/src/Kernel/FileManagedUnitTestBase.php

core/modules/file/tests/src/Kernel/LoadTest.php

core/modules/file/tests/src/Kernel/SaveTest.php

...es/file/tests/src/Kernel/Validation/FileValidatorTest.php

...sts/Drupal/KernelTests/Core/File/FileUrlGeneratorTest.php

.../tests/Drupal/KernelTests/Core/File/ScanDirectoryTest.php