test: replace concatenation with template literal

PR-URL: #14284
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: HSUCHING

test/parallel/test-http-upgrade-client2.js

@@ -6,9 +6,10 @@ const CRLF = '\r\n';
 
 const server = http.createServer();
 server.on('upgrade', function(req, socket, head) {
-  socket.write('HTTP/1.1 101 Ok' + CRLF +
-               'Connection: Upgrade' + CRLF +
-               'Upgrade: Test' + CRLF + CRLF + 'head');
+  socket.write(`HTTP/1.1 101 Ok${CRLF}` +
+               `Connection: Upgrade${CRLF}` +
+               `Upgrade: Test${CRLF}${CRLF}` +
+               'head');
   socket.on('end', function() {
     socket.end();
   });