selftests/btf: add VAR and DATASEC case for dedup tests

Add test case verifying that dedup happens (INTs are deduped in this
case) and VAR/DATASEC types are not deduped, but have their referenced
type IDs adjusted correctly.

Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Yonghong Song <yhs@fb.com>
Cc: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>