nixos/release: Use callSubTests for subtests

Should clean up a lot of these redundant lines for various sub-tests.

Note that the tests.boot* are now called tests.boot.boot*, but otherwise
all the test attribute names should stay the same.

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