Issue #3336578 by quietone, larowlan: Move migration tests to the Forum module

core/.cspell.json

core/modules/forum/tests/fixtures/drupal6.php

core/modules/forum/tests/fixtures/drupal7.php

...tional/migrate_drupal/d6/NoMultilingualReviewPageTest.php

...m/tests/src/Functional/migrate_drupal/d6/Upgrade6Test.php

...rate_drupal/d6/files/core/tests/fixtures/files/html-1.txt

...ate_drupal/d6/files/core/tests/fixtures/files/image-1.png

...ate_drupal/d6/files/core/tests/fixtures/files/image-2.jpg

..._drupal/d6/files/core/tests/fixtures/files/image-test.gif

..._drupal/d6/files/core/tests/fixtures/files/image-test.jpg

..._drupal/d6/files/core/tests/fixtures/files/image-test.png

...tional/migrate_drupal/d7/NoMultilingualReviewPageTest.php

...m/tests/src/Functional/migrate_drupal/d7/Upgrade7Test.php

...nal/migrate_drupal/d7/files/sites/default/files/cube.jpeg

...ional/migrate_drupal/d7/files/sites/default/files/ds9.txt

...igrate_drupal/d7/files/sites/default/private/Babylon5.txt

...m/tests/src/Kernel/Migrate/d6/MigrateForumConfigsTest.php

.../tests/src/Kernel/Migrate/d7/MigrateForumSettingsTest.php

...grate/tests/src/Kernel/Plugin/MigrationPluginListTest.php

...s/migrate_drupal/tests/src/Kernel/StateFileExistsTest.php