test: clean serializer files

app/src/test/java/sudoku/SudokuSerializerTest.java

@@ -1,5 +1,6 @@
 package sudoku;
 
+import java.io.File;
 import java.util.Random;
 
 import org.json.JSONObject;
@@ -25,6 +26,9 @@ public class SudokuSerializerTest {
         try {
             otherDoku = SudokuFactory.fromfile(savePath);
             assert (otherDoku != null);
+            // clean file after test
+            File fileToDelete = new File(savePath);
+            fileToDelete.delete();
         } catch (Exception e) {
             e.printStackTrace();
             assert false;