benchmark: add buffer.isUtf8 bench

PR-URL: https://github.com/nodejs/node/pull/54740
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

benchmark/buffers/buffer-isutf8.js

@@ -0,0 +1,23 @@
+'use strict';
+
+const common = require('../common.js');
+const buffer = require('node:buffer');
+const assert = require('node:assert');
+
+const bench = common.createBenchmark(main, {
+  n: [2e7],
+  length: ['short', 'long'],
+  input: ['regular string', '∀x∈ℝ: ⌈x⌉ = −⌊−x⌋'],
+});
+
+
+function main({ n, input, length }) {
+  const normalizedInput = length === 'short' ? input : input.repeat(300);
+  const encoder = new TextEncoder();
+  const buff = encoder.encode(normalizedInput);
+  bench.start();
+  for (let i = 0; i < n; ++i) {
+    assert.ok(buffer.isUtf8(buff));
+  }
+  bench.end(n);
+}