nixos/tests/vbox: Start X server with user "alice"

The VirtualBox tests so far ran the X server as root instead of user
"alice" and it did work, because we had access control turned off by
default.

Fortunately, it was changed in 1541fa35

.

As a side effect, it caused all the VirtualBox tests to fail because
they now can't connect to the X server, which is a good thing because
it's a bug of the VirtualBox tests.

So to fix it, let's just start the X server as user alice.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>