selftests/bpf: Add non-standardly sized enum tests for btf_dump

Add few custom enum definitions testing mode(byte) and mode(word)
attributes.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221212211505.558851-4-andrii@kernel.org