quieter_test() for UsbPlayer.quieter() implemented

src/test/java/device/UsbPlayerTest.java

@@ -7,20 +7,23 @@ import static org.assertj.core.api.Assertions.assertThat;
 
 class UsbPlayerTest {
 
-    UsbPlayer up =new UsbPlayer();
+    UsbPlayer up = new UsbPlayer();
 
 
     @Test
     void louder_test() {
-       up.setLautstaerke(40);
-       up.louder();
-       assertThat(up.getLautstaerke()).isEqualTo(41);
+        up.setLautstaerke(40);
+        up.louder();
+        assertThat(up.getLautstaerke()).isEqualTo(41);
     }
-/*
+
     @Test
-    void quiter() {
+    void quieter_test() {
+        up.setLautstaerke(40);
+        up.quieter();
+        assertThat(up.getLautstaerke()).isEqualTo(39);
     }
-
+/*
     @Test
     void getVolume() {
     }