tests: define tests via flake

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.

examples/README.md

@@ -55,6 +55,8 @@ The internal test suite is also useful for exploring features.\
 The following `run-tests.sh` commands leave no traces (outside of `/nix/store`) on
 the host system.
 
+`run-tests.sh` requires Nix >= 2.10.
+
 ```bash
 git clone https://github.com/fort-nix/nix-bitcoin
 cd nix-bitcoin/test
@@ -83,6 +85,27 @@ c systemctl status bitcoind
 ```
 See [`run-tests.sh`](../test/run-tests.sh) for a complete documentation.
 
+#### Flakes
+Tests can also be directly accessed via Flakes:
+```bash
+# Build test
+nix build --no-link ..#tests.default
+
+# Run a node in a VM. No tests are executed.
+nix run ..#tests.default.vm
+
+# Run a Python test shell inside a VM node
+nix run ..#tests.default.run -- --debug
+
+# Run a node in a container. Requires extra-container, systemd and root privileges
+nix run ..#tests.default.container
+nix run ..#tests.default.containerLegacy # For NixOS with `system.stateVersion` <22.05
+
+# Run a command in a container
+nix run ..#tests.default.container -- --run c nodeinfo
+nix run ..#tests.default.containerLegacy -- --run c nodeinfo # For NixOS with `system.stateVersion` <22.05
+```
+
 ### Real-world example
 Check the [server repo](https://github.com/fort-nix/nixbitcoin.org) for https://nixbitcoin.org
 to see the configuration of a nix-bitcoin node that's used in production.