Work around slowness in the KDE 5 test

The use of unionfs-fuse (57a0f14064)
slows down the KDE 5 test enough that it hits Hydra timeouts. (E.g. on
my laptop it went from ~5 min to ~30 min.) So disable it for the KDE
test.

http://hydra.nixos.org/build/45127422

nixos/tests/kde5.nix

@@ -18,6 +18,7 @@ import ./make-test.nix ({ pkgs, ...} :
       };
     };
     services.xserver.desktopManager.kde5.enable = true;
+    virtualisation.writableStore = false; # FIXME
   };
 
   enableOCR = true;