selftests/bpf: Test BTF_KIND_TAG for deduplication

Add unit tests for BTF_KIND_TAG deduplication for
  - struct and struct member
  - variable
  - func and func argument

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223052.248535-1-yhs@fb.com