Much of the lab work can be done with a resistor and a lab supply. Batteries can be used, but when one needs to check performance at high and low voltages the resistor plus power supply is more convenient and much quicker for a production line environment. Another possibility is a lab amplifier which will source and sink current; its output voltage can be varied and it will source or sink the current sort of like a battery though the voltage is fairly constant.
The resistor needs to be a small enough value so that the max charger current flows in the resistor when testing at low voltage, and the lab supply needs to give enough current to keep the voltage across the resistor constant at high voltage. For example, if the voltage range is 3V to 4.2V and the charge current is 3A, the resistor would be 1ohm, If the charge voltage is 4.2V, a constant current/constant voltage charger would be near 0A, so the lab supply would have to supply up to 4.2A. The lab supply voltage can be varied to simulate the range of battery voltages.