tests/basic: OCB large buffer check: make input buffer non-repeatable

* tests/basic.c (check_ocb_cipher_largebuf_split): Use SHA1 to
initialize input buffer.
(check_ocb_cipher): Update largebuf test vectors.

• GnuPG-bug-id: T5356
• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>