[gn] Automatically derive build configs in test runner.

tools/testrunner/utils/dump_build_config.py

+# Copyright 2016 the V8 project authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+"""Writes a dictionary to a json file with the passed key-value pairs.
+
+Expected to be called like:
+dump_build_config.py path/to/file.json [key1=value1 ...]
+
+The values are expected to be valid json. E.g. true is a boolean and "true" is
+the string "true".
+"""
+
+import json
+import os
+import sys
+
+assert len(sys.argv) > 1
+
+def as_json(kv):
+  assert '=' in kv
+  k, v = kv.split('=', 1)
+  return k, json.loads(v)
+
+with open(sys.argv[1], 'w') as f:
+  json.dump(dict(as_json(kv) for kv in sys.argv[2:]), f)