We differentiate between modules and baseModules in the
VM builder for NixOS tests. This way, nesting.children, eventhough
it doesn't inherit from parent, still has enough config to
actually complete the test. Otherwise, the qemu modules
would not be loaded, for example, and a nesting.children
statement would not evaluate.
This is actually very useful. Allows you to test switch-to-configuration
nesting.children is still currently still broken as it will throw
away 'too much' of the config, including the modules that make
nixos tests work in the first place. But that's something for
another time.