Add JUnit tests to test the behavior of the code (#24)

* Add some tests for Response and Task classes

src/test/java/duke/ResponseTest.java

@@ -0,0 +1,15 @@
+package duke;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class ResponseTest {
+    @Test
+    public void toString_success() {
+        assertEquals(
+                "    ____________________________________________________________" + System.lineSeparator()
+                + "     message" + System.lineSeparator()
+                + "    ____________________________________________________________"
+                , new Response("message").toString());
+    }
+}

src/test/java/duke/task/TaskTest.java

@@ -0,0 +1,29 @@
+package duke.task;
+
+import duke.Response;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class TaskTest {
+    private static String TASK_NOT_DONE_SYMBOL = " ";
+    private static String TASK_DONE_SYMBOL = "X";
+
+    @Test
+    public void toString_success() {
+        String taskDescription = "Lorem ipsum dolor sit amet";
+        Task task = new Task(taskDescription);
+        assertEquals(String.format("[%s] %s", TASK_NOT_DONE_SYMBOL, taskDescription), task.toString());
+        task.markDone();
+        assertEquals(String.format("[%s] %s", TASK_DONE_SYMBOL, taskDescription), task.toString());
+    }
+
+    @Test
+    public void getStatusIcon_success() {
+        String taskDescription = "Lorem ipsum dolor sit amet";
+        Task task = new Task(taskDescription);
+        assertEquals(TASK_NOT_DONE_SYMBOL, task.getStatusIcon());
+        task.markDone();
+        assertEquals(TASK_DONE_SYMBOL, task.getStatusIcon());
+    }
+}