NixOS test for `cardano-node`

[aciceri]

Probably this has to be done together with #27, or at least whoever works on #27 wants to implement a minimal test to check that the module he's hacking on really works as expected.

Take a look at what is done upstream, perhaps they already implemented NixOS modules.

The optimal solution would be starting a local network, which could be useful also for SPO-anywhere.

It would nice also writing an Hercules CI effect that run an overriden NixOS test derivation allowing the QEMU to access the internet (I believe that by default the VM uses a custom network, it's not online only because it's sandboxed).
Feel free to experiment this.

Also if you need you can split this issue into sub issues, in case please link them here as a to do list.

[brainrake]

fixed in #45