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.

.cirrus.yml

@@ -30,7 +30,7 @@ task:
       # This script is run as root
       build_script:
         - echo "sandbox = true" >> /etc/nix/nix.conf
-        - nix shell --inputs-from . nixpkgs#{bash,coreutils,gawk,cachix} -c ./test/ci/build.sh
+        - nix shell --inputs-from . nixpkgs#{bash,coreutils,cachix} -c ./test/ci/build.sh $scenario
 
     - name: flake
       build_script: