Logic for registering and checking properties of registrable classes was broken, allowing to ignore some errors. Recent fix rBeb798de101a `broke` the result of the pyapi_idprop_datablock test, because previously that test would fail (i.e. suceed, as it is an 'expected to break test') for a reason it was not designed to check. This is the problem with that kind of tests - you cannot really check that they are failing on the expected reason(s)...