test: change concatenated string to template

PR-URL: #16929
Reviewed-By: Vse Mozhet Byt <[email protected]>
Reviewed-By: Tobias Nießen <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Gireesh Punathil <[email protected]>
Reviewed-By: Franziska Hinkelmann <[email protected]>

test/fixtures/net-fd-passing-receiver.js

@@ -15,12 +15,12 @@ receiver = net.createServer(function(socket) {
     });
 
     passedSocket.on('data', function(data) {
-      passedSocket.send('[echo] ' + data);
+      passedSocket.send(`[echo] ${data}`);
     });
     passedSocket.on('close', function() {
       receiver.close();
     });
-    passedSocket.send('[greeting] ' + greeting);
+    passedSocket.send(`[greeting] ${greeting}`);
   });
 });