Test failures - MIGRATE_ACCESS_ADVANCED

Tests on d.o are failing, although they run fine for me locally. fields.inc references the permission MIGRATE_ACCESS_ADVANCED, which is defined by the migrate_ui module - when the tests run without migrate_ui enabled, they fail (I don't understand why they don't fail locally, though). Anyway, if we're going to reference the permission outside of migrate_ui, it needs to be defined in the core migrate module.