Refactor diffusion testnet simulation

The current module structure and some abstractions makes things unnecessarily
complicated. I think we should:

* merge parts of `Test.Ouroboros.Network.Diffusion.Node` and `Test.Ouroboros.Network.Testnet.Simulation.Node` which are used to run a single diffusion, exposing only the simpler interface (from the latter module);
* split Test.Ouroboros.Network.Testnet.Simulation.Node into two parts:

- `Test.Ouroboros.Network.Diffusion.Testnet` which runs a network
- `Test.Ouroboros.Network.Diffusion.Types` which includes `NodeArgs`, `SimArgs` and their arbitrary generators;

* use `Test.Ouroboros.Network.Diffusion` as the module which exposes all the
tests.