wp post edit should no longer call parent::update

This broke in f6285d3 when CommandWithDBObject::update was removed and
_update made protected. It should have been defined as $this->update
from the beginning.
Add feature tests for `wp post edit`

Adds two new feature tests for wp post edit: one that simulates opening
an editor and quitting right out of it, and one that simulates chnaging
a word and then saving.