Add failing unit tests for assignment errors.

src/test/java/dotsandboxes/DotsAndBoxesGridTest.java

@@ -27,5 +27,30 @@ public class DotsAndBoxesGridTest {
         assertTrue(true);
     }
 
-    // FIXME: You need to write tests for the two known bugs in the code.
+    @Test
+public void testSquareCompletion() {
+    DotsAndBoxesGrid grid = new DotsAndBoxesGrid(3, 3, 2);
+
+    // Draw only three lines of a box
+    grid.drawHorizontal(0, 0, 1);
+    grid.drawVertical(0, 0, 1);
+    grid.drawVertical(1, 0, 1);
+
+    // Check if the box is indeed incomplete
+    assertFalse(grid.boxComplete(0, 0));
+}
+
+@Test
+public void testDrawExistingLine() {
+    DotsAndBoxesGrid grid = new DotsAndBoxesGrid(3, 3, 2);
+
+    // Draw a line
+    grid.drawHorizontal(0, 0, 1);
+
+    // Attempt to draw the same line again. This should throw an IllegalStateException.
+    assertThrows(IllegalStateException.class, () -> {
+        grid.drawHorizontal(0, 0, 1);
+    });
+}
+
 }