selftests/bpf: Add a dedup selftest with equivalent structure types

Without previous libbpf patch, the following error will occur:

  $ ./test_progs -t btf
  ...
  do_test_dedup:FAIL:check btf_dedup failed errno:-22#13/205 btf/dedup: btf_type_tag #5, struct:FAIL

And the previous libbpf patch fixed the issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211115163943.3922547-1-yhs@fb.com