text: Title: typo in modeltest._on_rows_about_to_be_inserted
  Body: Hello,
  
  just to mention that, in modeltest._on_rows_about_to_be_inserted,  the following line (549):
  
  `last_data = self._model.data(last_index) if start - 1 > 0 else None`
  
  should be:
  
  `last_data = self._model.data(last_index) if start - 1 >= 0 else None`
  
  since start can be equal to 1.
  
  It is actually like this in the [qt5 source code](https://code.woboq.org/qt5/qtbase/src/testlib/qabstractitemmodeltester.cpp.html#_ZN31QAbstractItemModelTesterPrivate21rowsAboutToBeInsertedERK11QModelIndexii) (line 643):
  
  `c.last = (start - 1 >= 0) ? model->index(start - 1, 0, parent).data() : QVariant();`
  
  Thank you.
  