[FIX] test_assetsbundle: Properly unpatch ir.attachment

`_patch_method` and `_revert_method` do not work properly.
This commit fixes a test which patches `create` and `unlink`
of `ir.attachment`.
The methods are not correctly unpatched.

One consequence:
if a module is installed after this test is executed and
the new module overrides `create` and `unlink`, the overritten
methods are never called.

closes odoo/odoo#33952

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>