selftests/bpf: Add btf_dedup case with duplicated structs within CU

Add an artificial minimal example simulating compilers producing two
different types within a single CU that correspond to identical struct
definitions.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211117194114.347675-2-andrii@kernel.org