6 files declare their use of ThunderTestBase in Thunder 6.1.x

AdminToolbarTest.php in tests/src/Functional/Integration/AdminToolbarTest.php

ContentTranslationTest.php in tests/src/Functional/Integration/ContentTranslationTest.php

FilenameTransliterationTest.php in modules/thunder_media/tests/src/Functional/FilenameTransliterationTest.php

FileRemoveButtonTest.php in modules/thunder_media/tests/src/Functional/FileRemoveButtonTest.php

PasswordPolicyTest.php in tests/src/Functional/Integration/PasswordPolicyTest.php

RedirectTest.php in tests/src/Functional/Integration/RedirectTest.php