test: change string concatenation to template

PR-URL: #16919
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>

test/fixtures/print-10-lines.js

@@ -20,5 +20,5 @@
 // USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 for (var i = 0; i < 10; i++) {
-  console.log('count ' + i);
+  console.log(`count ${i}`);
 }