selftests/bpf: Add BTF_KIND_ENUM64 unit tests

Add unit tests for basic BTF_KIND_ENUM64 encoding.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062708.3724845-1-yhs@fb.com


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