Update test factories to use partner_name kwarg for partner name

Using "partner" is easily confused with passing an actual partner
instance.
Align options for Partner.name and order's Line.partner_name field

Partner name's are allowed to be empty, but Line.partner_name wasn't.
This caused orders to fail when the partner name was indeed empty, as
reported in #1206.

So the field options for both have to be identical. Unfortunately, that
means allowing null=True for Line.partner_name. But dropping null=True
from CharFields should be done as a separate effort all over Oscar's
codebase.

Fixes #1206.