selftests/bpf: Tests for btf_dedup_resolve_fwds

Tests to verify the following behavior of `btf_dedup_resolve_fwds`:
- remapping for struct forward declarations;
- remapping for union forward declarations;
- no remapping if forward declaration kind does not match similarly
  named struct or union declaration;
- no remapping if forward declaration name is ambiguous;
- base ids are considered for fwd resolution in split btf scenario.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Link: https://lore.kernel.org/bpf/20221109142611.879983-4-eddyz87@gmail.com