test: test mode passed as an options object in mkdir/mkdirSync

Add tests for mode passed as an options object in fs.mkdir() and
fs.mkdirSync().

This also adds coverage for mkdirSync() inside the conditional where
options.mode is not undefined.

PR-URL: #37008
Refs: https://coverage.nodejs.org/coverage-e3e054d020ee5ef6/lib/fs.js.html#L1023
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

test/parallel/test-fs-mkdir.js

@@ -53,6 +53,25 @@ function nextdir() {
   }));
 }
 
+// fs.mkdir creates directory with mode passed as an options object
+{
+  const pathname = path.join(tmpdir.path, nextdir());
+
+  fs.mkdir(pathname, { mode: 0o777 }, common.mustCall(function(err) {
+    assert.strictEqual(err, null);
+    assert.strictEqual(fs.existsSync(pathname), true);
+  }));
+}
+
+// fs.mkdirSync creates directory with mode passed as an options object
+{
+  const pathname = path.join(tmpdir.path, nextdir());
+
+  fs.mkdirSync(pathname, { mode: 0o777 });
+
+  assert.strictEqual(fs.existsSync(pathname), true);
+}
+
 // mkdirSync successfully creates directory from given path
 {
   const pathname = path.join(tmpdir.path, nextdir());