nixos/tests/networking: Expose subtests via attrs

So far the networking test expression only generated a single test
depending on the passed "test" attribute. This makes it difficult to
autodiscover the subtests with our shiny new callSubTests function.

This change essentially doesn't change the behaviour of the subtests but
rather exposes them as an attribute set instead of relying on a
particular input argument.

The useNetworkd argument still exists however.

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