TestApp: Add readStorageAddressBook()
TestApp: Add readStorageAddressBook()

Our system test needs to test that the storage file content is updated
correctly.

Let's add readStorageAddressBook() helper method, that returns the
address book created from the content of the storage file, to allow us
to verify that the file is correct.

damithc/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4,se-edu/addressbook-level3,CS2103R-Eugene-Peh/addressbook-level4,CS2103R-Eugene-Peh/addressbook-level4,damithc/addressbook-level4