test: replace string concatenation with template

PR-URL: #16913
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/fixtures/guess-hash-seed.js

@@ -134,7 +134,7 @@ const slow_str_gen = (function*() {
   let strgen_i = 0;
   outer:
   while (1) {
-    const str = '#' + (strgen_i++);
+    const str = `#${strgen_i++}`;
     for (let i = 0; i < 1000; i++) {
       if (time_set_lookup(tester_set, str) < tester_set_treshold)
         continue outer;