added test case for unmute() & and added productive code for RadioPlayer

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

@@ -201,7 +201,8 @@ public class RadioPlayer implements Device {
 
     @Override
     public String unmute() {
-        return null;
+        setLautstaerke(savedVolume);
+        return ("RadioPlayer is unmuted Volume is set to " + getVolume());
     }
 
 

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

@@ -365,4 +365,12 @@ class RadioPlayerTest {
                 Arguments.of("Test for mute RadioPlayer if volume is saved before muting", "saved", rp1, 30)
         );
     }
+    @Test
+    void TestUnmute() {
+        RadioPlayer rp = new RadioPlayer();
+        rp.setLautstaerke(40);
+        rp.mute();
+        rp.unmute();
+        assertThat(rp.getVolume()).describedAs("Test if unmute is setting the saved volume").isEqualTo(rp.savedVolume);
+    }
 }