PersistentStore: mkdir() for appDataDir (if necessary)
PersistentStore: mkdir() for appDataDir (if necessary)

Under normal circumstances, the app data directory is created in the setup
wizard.  However, it's useful to be able to create it for the tests.

(It's also not impossible that somebody might edit the config file manually,
and the error messages weren't very clear about what was wrong.)

shinnok/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,Tarsnap/tarsnap-gui,shinnok/tarsnap-gui,shinnok/tarsnap-gui