[test] Add test method required by d41832121b29

[ldapfeed] Fix crash when user's password has to be generated Binary isn't hashable hence can't be added to a set. Test added.

[test/ldapfeed] Drop unused constant

[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. Grafted from f87da59faea1 in default branch.