test: add test-buffer-prototype-inspect

lib/buffer.js defines Buffer.prototype.inspect() to override how buffers
are presented by util.inspect(). Add basic tests for it.

PR-URL: #11600
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

test/parallel/test-buffer-prototype-inspect.js

@@ -0,0 +1,23 @@
+'use strict';
+require('../common');
+
+// lib/buffer.js defines Buffer.prototype.inspect() to override how buffers are
+// presented by util.inspect().
+
+const assert = require('assert');
+const util = require('util');
+
+{
+  const buf = Buffer.from('fhqwhgads');
+  assert.strictEqual(util.inspect(buf), '<Buffer 66 68 71 77 68 67 61 64 73>');
+}
+
+{
+  const buf = Buffer.from('');
+  assert.strictEqual(util.inspect(buf), '<Buffer >');
+}
+
+{
+  const buf = Buffer.from('x'.repeat(51));
+  assert.ok(/^<Buffer (78 ){50}\.\.\. >$/.test(util.inspect(buf)));
+}