make test functions return void to drop warnings

and since we never returned anything
constify to get rid of more warnings
drop argc+argv to main since it's not used
drop unused ycfg structs
make functions static
use memcmp instead of strcmp
include yubikey.h to get function definitions we need
move yk_endian_swap_16 to ykcore.h since we export it anyways..
make const to avoid warning