dir-iterator: add tests for dir-iterator API

dir-iterator: add tests for dir-iterator API

Create t/helper/test-dir-iterator.c, which prints relevant information
about a directory tree iterated over with dir-iterator.

Create t/t0066-dir-iterator.sh, which tests that dir-iterator does
iterate through a whole directory tree as expected.

Signed-off-by: Daniel Ferreira <redacted>
[matheus.bernardino: update to use test-tool and some minor aesthetics]
Helped-by: Matheus Tavares <redacted>
Signed-off-by: Matheus Tavares <redacted>
Signed-off-by: Junio C Hamano <redacted>