gvfs-test: Fix private dir in generated smb.conf
gvfs-test: Fix private dir in generated smb.conf

Testing on Samba 3.6.9, "private directory" directive is not recognized
and smbd fails on secrets.tdb paths. This comes with Samba 4.0, however
it's still a synonym for "private dir" so let's use that.

https://bugzilla.gnome.org/show_bug.cgi?id=691568

gicmo/gvfs,gicmo/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,gicmo/gvfs,xkahn/gvfs-cmis,xkahn/gvfs-cmis,gicmo/gvfs,halfline/gvfs,halfline/gvfs,halfline/gvfs