[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.