This update adjusts internal tests so the Mexican e-invoicing module can be installed and checked on its own. It removes unused test setup and adds required tax data, helping reduce installation and validation issues without changing day-to-day user workflows.

- The variable `self.account_sale` is removed because it is not used in
the unit test.

- The tax `self.tax_positive` is created because it is originally created
in `l10n_mx` module