Add and use percentage constant (#32094)

* Add and use percentage constant

* Fix pylint error and broken test

homeassistant/components/linux_battery/sensor.py

@@ -6,7 +6,12 @@ from batinfo import Batteries
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import ATTR_NAME, CONF_NAME, DEVICE_CLASS_BATTERY
+from homeassistant.const import (
+    ATTR_NAME,
+    CONF_NAME,
+    DEVICE_CLASS_BATTERY,
+    UNIT_PERCENTAGE,
+)
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.entity import Entity
 
@@ -98,7 +103,7 @@ class LinuxBatterySensor(Entity):
     @property
     def unit_of_measurement(self):
         """Return the unit the value is expressed in."""
-        return "%"
+        return UNIT_PERCENTAGE
 
     @property
     def device_state_attributes(self):