selftests/bpf: Test BTF_KIND_ENUM64 for deduplication

Add a few unit tests for BTF_KIND_ENUM64 deduplication.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062713.3725409-1-yhs@fb.com


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