test: add filehandle sync() and datasync() tests

To increase test coverage for fs.promises,
added tests for filehandle.sync and filehandle.datasync.

PR-URL: #20530
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-fs-promises-file-handle-sync.js

@@ -0,0 +1,27 @@
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fixtures = require('../common/fixtures');
+const tmpdir = require('../common/tmpdir');
+
+const { access, copyFile, open } = require('fs').promises;
+const path = require('path');
+
+common.crashOnUnhandledRejection();
+
+async function validateSync() {
+  tmpdir.refresh();
+  const dest = path.resolve(tmpdir.path, 'baz.js');
+  await copyFile(fixtures.path('baz.js'), dest);
+  await access(dest, 'r');
+  const handle = await open(dest, 'r+');
+  await handle.datasync();
+  await handle.sync();
+  const buf = Buffer.from('hello world');
+  await handle.write(buf);
+  const ret = await handle.read(Buffer.alloc(11), 0, 11, 0);
+  assert.strictEqual(ret.bytesRead, 11);
+  assert.deepStrictEqual(ret.buffer, buf);
+}
+
+validateSync();