added first test case for changeToSavedPlaylist() & and added productive code

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

@@ -202,4 +202,7 @@ public class RadioPlayer implements Device {
         } else return " Last Station: 01 " + station + " can´t by deleted";
     }
 
+    public String changeToSavedPlaylist() {
+        return "You are already in saved Playlist!";
+    }
 }

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

@@ -244,4 +244,21 @@ class RadioPlayerTest {
                 Arguments.of("Test for not delete station in saved playlist if this is the last station", rp1, "noDelete", true)
         );
     }
+
+    @ParameterizedTest
+    @MethodSource("changeToSavedPlaylistOptions")
+    void testChangeToSavedPlaylist(String testName, RadioPlayer testRp, String expectedResult) {
+        String savedStation = testRp.playedStation;
+        assertThat(savedStation).describedAs(testName).isEqualTo(expectedResult);
+    }
+
+    static Stream<Arguments> changeToSavedPlaylistOptions() {
+        RadioPlayer rp = new RadioPlayer();
+        rp.changeToSavedPlaylist();
+
+        return Stream.of(
+                Arguments.of("Test for change to saved playlist when you already in saved playlist", rp, "YouFM")
+
+        );
+    }
 }