Improved 'testing' task based on feedback

Introduction to Objects/Testing/Exercise 1/task-info.yaml

@@ -9,6 +9,9 @@ files:
   - offset: 159
     length: 17
     placeholder_text: h / 2 * (x + y)
+  - offset: 257
+    length: 116
+    placeholder_text: // add more tests
 - name: test/Tests.kt
   visible: false
 feedback_link: |

Introduction to Objects/Testing/Exercise 1/task.md

@@ -1,5 +1,7 @@
 ## Testing (#1)
 
-Use Test Driven Development (write a failing test, and then write the code to
-fix it) to calculate the area of a quadrangle. Start with the sample code and
-fix the intentional bugs.
+We'll use Test Driven Development (first writing a failing test,
+and then writing the code to fix it) to calculate the area of a quadrangle.
+
+Some tests are already written, your task is to add more.
+Start with the sample code and fix the intentional bugs.