[#208] DeveloperGuide: Change setup to use default test configuration (#226)

The working directory needs to be changed for the tests to run properly.
Therefore, the developer guide instructs the developer to modify the
generated test configuration.

If the IDE generates another new test configuration, the settings needs
to be changed again, otherwise the tests will fail again.

Let's change the instructions to ask the developers to modify the
default test configuration instead of the generated one.

Author: yamgent

doc/DeveloperGuide.md

@@ -28,9 +28,10 @@
 10. Run all the tests (right-click the `test` folder, and click `Run 'All Tests'`)
 11. Observe how some tests fail. That is because they try to access the test data from the wrong directory (the working directory is expected to be the root directory, but IntelliJ runs the test with `test\` as the working directory by default). To fix this issue:
     1. Go to `Run` -> `Edit Configurations...`
-    2. On the list at the left, ensure that `All in test` is selected
-    3. Under `Configuration`, change the `Working directory` to the `addressbook-level2` folder
-    4. Click `OK`
+    2. On the list at the left, expand `JUnit`, and remove all the test configurations (e.g. `All in test`) by selecting it and clicking on the '-' icon at the top of the list
+    3. Expand `Defaults`, and ensure that `JUnit` is selected
+    4. Under `Configuration`, change the `Working directory` to the `addressbook-level2` folder
+    5. Click `OK`
 12. Run the tests again to ensure they all pass now.
 
 ## Design