test: refactor test-fs-fsync

- replace var with const.
- remove successes var.
- use assert.ifError() for handling all errors.
- wrap all callbacks with common.mustCall().

PR-URL: https://github.com/nodejs/node/pull/10176
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

test/parallel/test-fs-fsync.js

@@ -1,32 +1,23 @@
 'use strict';
-var common = require('../common');
-var assert = require('assert');
+const common = require('../common');
+const assert = require('assert');
 
-var path = require('path');
-var fs = require('fs');
-var successes = 0;
+const path = require('path');
+const fs = require('fs');
 
-var file = path.join(common.fixturesDir, 'a.js');
+const file = path.join(common.fixturesDir, 'a.js');
 
-fs.open(file, 'a', 0o777, function(err, fd) {
-  if (err) throw err;
+fs.open(file, 'a', 0o777, common.mustCall(function(err, fd) {
+  assert.ifError(err);
 
   fs.fdatasyncSync(fd);
-  successes++;
 
   fs.fsyncSync(fd);
-  successes++;
 
-  fs.fdatasync(fd, function(err) {
-    if (err) throw err;
-    successes++;
-    fs.fsync(fd, function(err) {
-      if (err) throw err;
-      successes++;
-    });
-  });
-});
-
-process.on('exit', function() {
-  assert.equal(4, successes);
-});
+  fs.fdatasync(fd, common.mustCall(function(err) {
+    assert.ifError(err);
+    fs.fsync(fd, common.mustCall(function(err) {
+      assert.ifError(err);
+    }));
+  }));
+}));