[nextion] Brightness control tweaks (#8027)

tests/components/nextion/common.yaml

@@ -2,6 +2,8 @@ esphome:
   on_boot:
     - lambda: 'ESP_LOGD("display","is_connected(): %s", YESNO(id(main_lcd).is_connected()));'
 
+    - display.nextion.set_brightness: 80%
+
     # Binary sensor publish action tests
     - binary_sensor.nextion.publish:
         id: r0_sensor