nixos: Add VM test for i3 window manager.

Currently just makes sure that by default it's possible to open a
terminal.

And exactly this should be the main point that might confuse users of i3
in NixOS, because i3 doesn't print a warning/error if it is unable to
start the terminal emulator.

Thanks to @waaaaargh for reporting this issue.

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