nixos/tests/virtualbox: Split up subtests

Now subtests are separate derivations, because the individual tests do
not depend on state from previous test runs.

This has the advantage that it's easier to run individiual tests and
it's also easier to pinpoint individual tests that randomly fail.

I ran all of these tests locally and they still succeed.

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