[FIX] payment_paypal: fix tests

Since changes made in this commit https://github.com/odoo/odoo/commit/fc1389f8d881adfb94ea1b784fb7104b9eee5d2d

 the value of field 'custom'
is now set and wasn't before.

closes odoo/odoo#32258

Signed-off-by: Toufik Benjaa (tbe) <tbe@odoo.com>