15 files declare their use of MigrateDrupalTestBase in Drupal 9

DestinationCategoryTest.php in core/modules/migrate_drupal/tests/src/Kernel/Plugin/migrate/DestinationCategoryTest.php

FilterFormatPermissionTest.php in core/modules/filter/tests/src/Kernel/Migrate/d6/FilterFormatPermissionTest.php

MigrateAggregatorStubTest.php in core/modules/aggregator/tests/src/Kernel/Migrate/MigrateAggregatorStubTest.php

MigrateBlockContentStubTest.php in core/modules/block_content/tests/src/Kernel/Migrate/MigrateBlockContentStubTest.php

MigrateCommentStubTest.php in core/modules/comment/tests/src/Kernel/Migrate/MigrateCommentStubTest.php

MigrateDrupal6TestBase.php in core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6TestBase.php

MigrateDrupal7TestBase.php in core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7TestBase.php

MigrateFileStubTest.php in core/modules/file/tests/src/Kernel/Migrate/MigrateFileStubTest.php

MigrateMenuLinkContentStubTest.php in core/modules/menu_link_content/tests/src/Kernel/Migrate/MigrateMenuLinkContentStubTest.php

MigrateNodeStubTest.php in core/modules/node/tests/src/Kernel/Migrate/MigrateNodeStubTest.php

MigrateShortcutStubTest.php in core/modules/shortcut/tests/src/Kernel/Migrate/MigrateShortcutStubTest.php

MigrateTaxonomyTermStubTest.php in core/modules/taxonomy/tests/src/Kernel/Migrate/MigrateTaxonomyTermStubTest.php

MigrateUserStubTest.php in core/modules/user/tests/src/Kernel/Migrate/MigrateUserStubTest.php

MigrationLabelExistTest.php in core/modules/migrate_drupal_ui/tests/src/Kernel/MigrationLabelExistTest.php

MigrationProvidersExistTest.php in core/modules/migrate/tests/src/Kernel/Plugin/MigrationProvidersExistTest.php