refactor: clean up test output and improve code formatting

app/src/test/java/sudoku/TestSudokusString.java

@@ -9,7 +9,7 @@ public class TestSudokusString {
     @Test
     public void sudokuString() {
         System.out.println("TEST SUDOKU STRING : ");
-        
+
         // Create a new Sudoku
         Sudoku sudoku = new Sudoku(9);