test: replace string concatenation with template

PR-URL: #15915
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-http-incoming-pipelined-socket-destroy.js

@@ -60,9 +60,10 @@ const server = http.createServer(common.mustCall(function(req, res) {
 
 // Make a bunch of requests pipelined on the same socket
 function generator(seeds) {
+  const port = server.address().port;
   return seeds.map(function(r) {
-    return 'GET /' + r + ' HTTP/1.1\r\n' +
-           `Host: localhost:${server.address().port}\r\n` +
+    return `GET /${r} HTTP/1.1\r\n` +
+           `Host: localhost:${port}\r\n` +
            '\r\n' +
            '\r\n';
   }).join('');