DeveloperGuide: update Testing section
DeveloperGuide: update Testing section

In particular, UrlUtilTest does not exist anymore.

Also, the `guitests` package does not exist anymore. We use the
seedu.address.testutil.GuiTests JUnit test category instead.

CS2103AUG2016-T11-C4/main,nightism/addressbook-level4,snehasp13/main,CS2103AUG2016-T11-C4/main,snehasp13/main,nightism/addressbook-level4