chore(tests): update spell-checker ignore list in test_readlink.rs

Add 'parentdir' to the ignored words to suppress spell-checker warnings, as it's used in test scenarios and not a misspelled term.

Author: mattsu2020

tests/by-util/test_readlink.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 //
-// spell-checker:ignore regfile
+// spell-checker:ignore regfile parentdir
 
 use uutests::util::{TestScenario, get_root_path};
 use uutests::{at_and_ucmd, new_ucmd, path_concat, util_name};