Support customTestFor with multiple (different) nodes

[Sohalt]

The tests defined in nixos.hosts.<hostname>.tests currently import the host's configuration on each machine defined in the test (using extraConfigurations).

This is unfortunate when trying to test servers for example, where it would be nice to have a second machine act as a client (without the configuration of the host under test).

It would be cool to have the possibility to specify additional nodes with an entirely custom configuration.

It would be even better, if there was a way to test multiple hosts in conjunction, e.g. if I have two machines defined in hosts/server/default.nix and hosts/client/default.nix I'd love to be able to test them together, referring to them by their hostname.

[Sohalt]

I tried to make progress on this front to no avail. Anyone interested in pairing on this?