test: refactor test-fs-buffer

* Remove unneeded temp dir cleanup
* Add check for error in `.close()` callback
* Improve error reporting

On that last bullet point, the previous version of the test reported
errors like this:

```
AssertionError: [ '.empty-repl-history-file',
  '.node_repl_history',
  'GH-1899-output.js',
  'GH-892-request.js',
  'a.js',
  'a1.js',
  'agen deepStrictEqual [ '.empty-repl-history-file',
  '.node_repl_history',
  'GH-1899-output.js',
  'GH-892-request.js',
  'a.js',
  'a1.js',
  'agen
```

Now, they look like this:

```
AssertionError: expected *, got ! by hex decoding 2a
```

PR-URL: https://github.com/nodejs/node/pull/11232


Reviewed-By: James M Snell <jasnell@gmail.com>