power: supply: test-power: Test access to extended power supply

Test that power supply extensions can access properties of their
power supply using power_supply_get_property_direct(). This both
ensures that the functionality works and serves as an example for
future driver developers.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Acked-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Link: https://lore.kernel.org/r/20250627205124.250433-2-W_Armin@gmx.de


Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>