test: add missing line breaks to keep-alive header of slow headers test

PR-URL: #27442
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-http-slow-headers-keepalive.js

@@ -8,7 +8,7 @@ const { finished } = require('stream');
 const headers =
   'GET / HTTP/1.1\r\n' +
   'Host: localhost\r\n' +
-  'Connection: keep-alive' +
+  'Connection: keep-alive\r\n' +
   'Agent: node\r\n';
 
 let sendCharEvery = 1000;