test: replace string concatenation with template

Replace string concatenation with template literals in
test/async-hooks/verify-graph.js.

PR-URL: #14279
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>

test/async-hooks/verify-graph.js

@@ -106,7 +106,7 @@ module.exports.printGraph = function printGraph(hooks) {
   function procesNode(x) {
     const key = x.type.replace(/WRAP/, '').toLowerCase();
     if (!ids[key]) ids[key] = 1;
-    const id = key + ':' + ids[key]++;
+    const id = `${key}:${ids[key]++}`;
     uidtoid[x.uid] = id;
     const triggerAsyncId = uidtoid[x.triggerAsyncId] || null;
     graph.push({ type: x.type, id, triggerAsyncId });