[3.11] gh-109845: Make test_ftplib more stable under load (GH-109912) (GH-109920)

recv() can return partial data cut in the middle of a multibyte
character. Test raw binary data instead of data incorrectly decoded by parts.
(cherry picked from commit 2ef2fffe3be953b91852585c75188d5475b09474)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>