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.sh

@@ -0,0 +1,50 @@
+#!/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
+
+testDriver=$1
+shift
+
+# Variable 'tests' contains the Python code that is executed by the driver on startup
+if [[ ${1:-} == --debug ]]; then
+    shift
+    echo "Running interactive testing environment"
+    # Start REPL.
+    # Use `code.interact` for the REPL instead of the builtin test driver REPL
+    # because it supports low featured terminals like Emacs' shell-mode.
+    tests='
+is_interactive = True
+exec(open(os.environ["testScript"]).read())
+if "machine" in vars(): machine.start()
+import code
+code.interact(local=globals())
+'
+    echo
+    echo "Starting VM, data dir: $dataDir"
+else
+    tests='exec(open(os.environ["testScript"]).read())'
+fi
+
+if [[ ! ${NIX_BITCOIN_VM_ENABLE_NETWORK:-} ]]; then
+    QEMU_NET_OPTS='restrict=on'
+fi
+
+# The VM creates a VDE control socket in $PWD
+env --chdir "$dataDir" -i \
+    USE_TMPDIR=1 \
+    TMPDIR="$dataDir" \
+    QEMU_OPTS="-nographic ${QEMU_OPTS:-}"  \
+    QEMU_NET_OPTS="${QEMU_NET_OPTS:-}" \
+    "$testDriver/bin/nixos-test-driver" <(echo "$tests") "$@"