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.

test/ci/build.sh

@@ -1,12 +1,14 @@
 #!/usr/bin/env bash
 
 # This script can also be run locally for testing:
-#   scenario=default ./build.sh
+#   ./build.sh <scenario>
 #
 # When variable CIRRUS_CI is unset, this script leaves no persistent traces on the host system.
 
 set -euo pipefail
 
+scenario=$1
+
 if [[ -v CIRRUS_CI ]]; then
     if [[ ! -e /dev/kvm ]]; then
         >&2 echo "No KVM available on VM host."
@@ -16,5 +18,5 @@ if [[ -v CIRRUS_CI ]]; then
     chmod o+rw /dev/kvm
 fi
 
-# shellcheck disable=SC2154
-"${BASH_SOURCE[0]%/*}/../run-tests.sh" --ci --scenario "$scenario"
+cd "${BASH_SOURCE[0]%/*}"
+exec ./build-to-cachix.sh --expr "(builtins.getFlake (toString ../..)).legacyPackages.\${builtins.currentSystem}.tests.$scenario"