benchmark: change iterations in benchmark/es/string-concatenations.js

Increase the number of iterations from `1e3` to `1e6`
to avoid the test performance gap caused by inactive
V8 optimization caused by too few iterations.

Fixes: https://github.com/nodejs/node/issues/50571
PR-URL: https://github.com/nodejs/node/pull/50585
Reviewed-By: Vinícius Lourenço Claro Cardoso <contact@viniciusl.com.br>
Reviewed-By: Debadree Chatterjee <debadree333@gmail.com>

benchmark/es/string-concatenations.js

@@ -3,7 +3,7 @@
 const common = require('../common.js');
 
 const configs = {
-  n: [1e3],
+  n: [1e6],
   mode: [
     'multi-concat',
     'multi-join',