selftests/bpf: Add few corner cases to test padding handling of btf_dump

Add few hand-crafted cases and few randomized cases found using script
from [0] that tests btf_dump's padding logic.

  [0] https://lore.kernel.org/bpf/85f83c333f5355c8ac026f835b18d15060725fcb.camel@ericsson.com/



Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20221212211505.558851-7-andrii@kernel.org