The test assumes you don't have any other move with taxes in your DB,
which can be happen sometimes, so we need to change assert for being
compatible with this situation.
* Tests
* PEP8
* Use invoice._convert_to_write(invoice._cache). This way, the onchange will be inheritable and will add here also the added values
* better get_context_values
* unify method compute_balance
* open move lines linked to balance