quieter_test() for UsbPlayer.quieter() implemented

src/test/java/device/UsbPlayerTest.java

@@ -16,11 +16,14 @@ class UsbPlayerTest {
         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() {
     }