test: cover 'close' method (in Dir class) with tests

Add 2 tests for full covering of method 'close' in class Dir

1. If pass smth that not string as a callback - throw an exception
2. If do .close() on already closed directory - throw an exception

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines