[migration] Test and fix case of addition of an attribute with some serialized constraint It was crashing on attempt to remove an unexisting constraint. This code may be removed safely as it's expected to be handled by the constraint removal hook. Notice that adding a vocabulary on shortpara makes yams change the varchar max size. This is arguable but not the point of this test, so simply adapt the max size defined in the schema coherently so we don't have to bother with that at this point.

[test] Add a migration test for update of a static vocabulary constraint This is a constraint serialized to be checked by the DB backend, hence ensure the old constraint is dropped and the new one is created.