selftests/bpf: Add test for libbpf_sha256()

Test that libbpf_sha256() calculates SHA-256 digests correctly.

Tested with:
    make -C tools/testing/selftests/bpf/
    ./tools/testing/selftests/bpf/test_progs -t sha256 -v

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>