nixos/release.nix: Add a callSubTests function

This should de-clutter the various redundant lines of callTest's on
subtests so that every main test file should have only one line with a
callSubTests instead.

Overrides work the same way as callTest, except that if the system
attribute is explicitly specified we do not generate attributes for all
available systems.

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