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

Now, they look like this:

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

PR-URL: nodejs#11232
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-fs-buffer.js

@@ -18,8 +18,8 @@ assert.doesNotThrow(() => {
  fs.open(buf, 'w+', common.mustCall((err, fd) => {
  assert.ifError(err);
  assert(fd);
- fs.close(fd, common.mustCall(() => {
- fs.unlinkSync(buf);
+ fs.close(fd, common.mustCall((err) => {
+ assert.ifError(err);
  }));
  }));
 });
@@ -29,13 +29,17 @@ assert.throws(() => {
 }, /path must be a string or Buffer/);
 
 const dir = Buffer.from(common.fixturesDir);
-fs.readdir(dir, 'hex', common.mustCall((err, list) => {
+fs.readdir(dir, 'hex', common.mustCall((err, hexList) => {
  assert.ifError(err);
- list = list.map((i) => {
- return Buffer.from(i, 'hex').toString();
- });
- fs.readdir(dir, common.mustCall((err, list2) => {
+ fs.readdir(dir, common.mustCall((err, stringList) => {
  assert.ifError(err);
- assert.deepStrictEqual(list, list2);
+ stringList.forEach((val, idx) => {
+ const fromHexList = Buffer.from(hexList[idx], 'hex').toString();
+ assert.strictEqual(
+ fromHexList,
+ val,
+ `expected ${val}, got ${fromHexList} by hex decoding ${hexList[idx]}`
+ );
+ });
  }));
 }));