Tests: First VFS tests

There are three tests to test the VFS:

 1. "plain": just open and close the DB
 2. "data": open db, insert some data, close db and then ensuring
    the data reopening
 3. "encrypted:" The same as "data", but encrypting the database.
    To do it, we create a tests keys in order to encrypt and sign
    the DB.
    This is something to improve, because the keys creation takes
    a long time, and the GPGme backend request for confirmation
    when we delete the keys from the public ring.

https://bugzilla.gnome.org/show_bug.cgi?id=659500