tests/basic: silence GCC-8 warning

* tests/basic.c (check_ofb_cipher, check_stream_cipher): Change
tv[].data[].inlen type from signed to unsigned integer.

Patch silences new GCC-8 compiler warning:
'__builtin_memcmp_eq' specified size between 18446744071562067968 and
18446744073709551615 exceeds maximum object size 9223372036854775807
[-Wstringop-overflow=]

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