Add comprehensive test harness with env var support

Add test/test_proxy_headers.js with full feature parity to Python:
- PROXY_URL, TEST_URL for basic configuration
- PROXY_HEADER for checking specific response header
- SEND_PROXY_HEADER, SEND_PROXY_VALUE for sending headers to proxy
- -v/--verbose, -l/--list, -h/--help flags
- Tests for all supported libraries: core, axios, node-fetch, got, undici, superagent

Made-with: Cursor

Author: proxymeshai

package.json

@@ -13,7 +13,8 @@
     "./superagent": "./lib/superagent-proxy.js"
   },
   "scripts": {
-    "test": "node test/run_tests.js",
+    "test": "node test/test_proxy_headers.js",
+    "test:verbose": "node test/test_proxy_headers.js -v",
     "lint": "eslint lib test"
   },
   "keywords": [