added first test case for mute() & and added productive code for RadioPlayer

src/main/java/device/radioPlayer/RadioPlayer.java

@@ -193,7 +193,8 @@ public class RadioPlayer implements Device {
 
     @Override
     public String mute() {
-        return null;
+        setLautstaerke(0);
+        return "RadioPlayer is muted now";
     }
 
     @Override

src/test/java/device/radioPlayer/RadioPlayerTest.java

@@ -342,4 +342,22 @@ class RadioPlayerTest {
                 Arguments.of("Test for return regional playlist", rp1, "region", rp1.regionPlaylist.toArray(new String[0]))
         );
     }
+
+    @ParameterizedTest
+    @MethodSource("muteOptions")
+    void testMute(String testName, RadioPlayer testRp, int expectedResult) {
+
+        int volume = testRp.getVolume();
+        assertThat(volume).describedAs(testName).isEqualTo(expectedResult);
+    }
+
+    static Stream<Arguments> muteOptions() {
+        RadioPlayer rp = new RadioPlayer();
+        rp.setLautstaerke(25);
+        rp.mute();
+
+        return Stream.of(
+                Arguments.of("Test for mute RadioPlayer if volume is actually 0", rp, 0)
+        );
+    }
 }