Advantages:
- Pure test evaluations
- The test framework can now be used by flakes that extend nix-bitcoin
- Most features of `run-tests.sh` are now accessible via `nix build`/`nix run`.
We keep `run-tests.sh` for advanced features like `scenarioOverridesFile` and adhoc scenarios.
Other changes:
- `run-tests.sh` now builds aggregate VM tests like `basic` or
`buildable` by creating all VMs in a single evaluation.
This speeds up the tests and eases debugging by separating the eval and build steps.
- Use the new `nix` CLI which has improved build output logging
by prefixing output lines with the origin drv name.
