[FIX] account: Reintroduce the mocked_today method

This method has been lost during the forward-port of:
https://github.com/odoo/odoo/commit/161498cf8a75d62f9a8e1a17c6dc62a142b85acc



closes odoo/odoo#52424

X-original-commit: 18d08cc3b48a12c015bd2513cc64ee1166336b5d
Related: odoo/enterprise#10956
Signed-off-by: Josse Colpaert <jco@openerp.com>