6 files declare their use of MigrateTestBase in Drupal 9

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

MigrateDrupalTestBase.php in core/modules/migrate_drupal/tests/src/Kernel/MigrateDrupalTestBase.php

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

MigrationSourceCacheTest.php in core/modules/migrate/tests/src/Kernel/Plugin/source/MigrationSourceCacheTest.php

NodeMigrationTypePluginAlterTest.php in core/modules/migrate_drupal/tests/src/Kernel/NodeMigrationTypePluginAlterTest.php

SqlTest.php in core/modules/migrate/tests/src/Kernel/Plugin/id_map/SqlTest.php