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/lib/run-vm-without-tests.sh

@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# This script uses the following env vars:
+# NIX_BITCOIN_VM_ENABLE_NETWORK
+# NIX_BITCOIN_VM_DATADIR
+# QEMU_OPTS
+# QEMU_NET_OPTS
+
+if [[ ${NIX_BITCOIN_VM_DATADIR:-} ]]; then
+    dataDir=$NIX_BITCOIN_VM_DATADIR
+else
+    dataDir=$(mktemp -d /tmp/nix-bitcoin-vm.XXX)
+    trap 'rm -rf "$dataDir"' EXIT
+fi
+
+if [[ ! ${NIX_BITCOIN_VM_ENABLE_NETWORK:-} ]]; then
+    QEMU_NET_OPTS='restrict=on'
+fi
+
+# TODO-EXTERNAL:
+# Pass PATH because run-*-vm is impure (requires coreutils from PATH)
+env -i \
+    PATH="$PATH" \
+    USE_TMPDIR=1 \
+    TMPDIR="$dataDir" \
+    NIX_DISK_IMAGE="$dataDir/img.qcow2" \
+    QEMU_OPTS="${QEMU_OPTS:-}" \
+    QEMU_NET_OPTS="${QEMU_NET_OPTS:-}" \
+    "${BASH_SOURCE[0]%/*}"/run-*-vm