[FIX] purchase_mrp: set anglo_saxon flag in the test

If the module `l10n_de` is installed, the test
`test_kit_anglo_saxo_price_diff` will failed. This is because the flag
`use_anglo_saxon` is not enabled by default in the chart template of
German companies. Therefore, when posting the invoice, we skip the
anglo-saxon lines generation:
https://github.com/odoo/odoo/blob/f3ae759f2d54829f91badd32cacf70e8a8211288/addons/purchase_stock/models/account_invoice.py#L40-L42


This explains why the test fails.

OPW-2843861

closes odoo/odoo#94421

Signed-off-by: Adrien Widart <awt@odoo.com>