On-Board Charger Battery Load
To measure power capabilities of charger:
Yes, the resistors will be enough, because they will give you the opportunity to load the charger with the same current as in the real application.
EDIT: And as I just saw in the comments to the question: Yes, estimation of power efficiency will also be possible.

To measure behaviour during real charging process:
No, the resistors won't emulate a real charging process but be just a constant load. The charger won't be able to detect a progress in charging state and can't adjust to that.  

Does your lab have a programmable electronic load? If yes, you can use that to simulate the charging process of the batteries and by that verify the behaviour of the charger.