test: replace concatenation with template literals

Use template literals instead of string concatenation in
test/parallel/test-http-extra-response.js

PR-URL: #14296
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: David Cai <davidcai1993@yahoo.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-extra-response.js

@@ -32,7 +32,7 @@ const net = require('net');
 const body = 'hello world\r\n';
 const fullResponse =
     'HTTP/1.1 500 Internal Server Error\r\n' +
-    'Content-Length: ' + body.length + '\r\n' +
+    `Content-Length: ${body.length}\r\n` +
     'Content-Type: text/plain\r\n' +
     'Date: Fri + 18 Feb 2011 06:22:45 GMT\r\n' +
     'Host: 10.20.149.2\r\n' +