power: supply: test-power: implement charge_behaviour property

To validate the special formatting of the "charge_behaviour" sysfs
property add it to the example driver.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20240306-power_supply-charge_behaviour_prop-v3-1-d04cf1f5f0af@weissschuh.net


Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>