kunit: Add tests for csum_ipv6_magic and ip_fast_csum

Supplement existing checksum tests with tests for csum_ipv6_magic and
ip_fast_csum.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Link: https://lore.kernel.org/r/20240108-optimize_checksum-v15-5-1c50de5f2167@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>