test_power: Add CHARGE_COUNTER properties

CHARGE_COUNTER is really general in other power supply drivers and
Android also has an interface to monitor CHARGE_COUNTER, so let's
add it into test framework.

Set default as -1000 is because the default status is
POWER_SUPPLY_STATUS_DISCHARGING, which means the counter should be
negative, and 1000 means not zero but small enough.

Signed-off-by: Lecopzer Chen <lecopzer.chen@mediatek.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>