TestUtil: Update access modifier of SANDBOX_FOLDER
TestUtil: Update access modifier of SANDBOX_FOLDER

Constant SANDBOX_FOLDER has public access modifier even though it is
only called within the class.

This unnecessarily exposes it to other classes.

Let’s change its access modifier to private.

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