selftests/bpf: Test BTF_KIND_DECL_TAG for deduplication

Add BTF_KIND_TYPE_TAG duplication unit tests.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211112012635.1506853-1-yhs@fb.com