[nus-cs2103-AY1617S1#89] Test folder: add addressbook.txt (nus-cs2103…

…-AY1617S1#90)

The default storage file 'addressbook.txt' is not provided by default.
The program has to create a new storage file, and this process is 
written in the program's log.

Due to the additional information in the log, the first test run after 
cloning the master always result in failure, as the additional
information creates a difference between actual.txt and expected.txt,
causing the test to fail.

Let's add the default storage file 'addressbook.txt' into the test
folder so that the runtests.bat will stop failing on first run.