Because nesting.clone calls 'eval-config.nix' manually,
without the 'extraArgs' argument that provides the 'nodes'
argument to nixos modules in nixos tests, evaluating
of 'nesting.clone' definitions would fail with the following error

while evaluating the module argument `nodes' in "<redacted>"
while evaluating the attribute '_module.args.nodes' at undefined position:
attribute 'nodes' missing, at <redacted./nixpkgs/lib/modules.nix:163:28

by not using 'extraArgs' but a nixos module instead, the nodes parameter
gets propagated to the 'eval-config.nix' call that  'nesting.clone'
makes too -  getting rid of the error.

See  https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/system/activation/top-level.nix#L13-L23
See  https://github.com/NixOS/nixpkgs/blob/master/nixos/lib/build-vms.nix#L27
See  NixOS#20886 (comment)

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.

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.