tests/basic: add testing for partial bulk processing code paths
* tests/basic.c (check_one_cipher_core): Add 'split_mode' parameter and handling for split_mode==1. (check_one_cipher): Use split_mode==0 for existing check_one_cipher_core calls; Add new large buffer check with split_mode==1.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>