fix build

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

@@ -2,6 +2,8 @@ package sudoku;
 
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
+import java.util.Random;
+
 import org.junit.jupiter.api.Test;
 
 import sudoku.io.SudokuPrinter;
@@ -19,9 +21,13 @@ public class SudokuSerializerTest {
 
     @Test
     void testSerialize() {
-        int blockWidth = 3;
-        int blockHeight = 3;
-        testSerializeWithSize(blockWidth, blockHeight);
+        int testCount = 5;
+        Random r = new Random();
+        for (int i = 0; i < testCount; i++) {
+            int blockWidth = r.nextInt(20) + 1;
+            int blockHeight = r.nextInt(20) + 1;
+            testSerializeWithSize(blockWidth, blockHeight);
+        }
     }
 
 }