was wrongly named JSONSerializationSchemaTest beforehand (due to
copy-paste from json test)

apparently, the schema verification does not let you add the
"schemaLocation" attribute by hand to your rootElement. Instead, the
suggested way is to use a etree.QName

also, change every usage of this function

To avoid duplicate code, add a function generate_parent which will be
used to generate a parent element and immediatly insert the serialized
info from abstract classes