selftests/bpf: Test btf dump for struct with padding only fields

Structures with zero regular fields but some padding constitute a
special case in btf_dump.c:btf_dump_emit_struct_def with regards to
newline before closing '}'.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221001104425.415768-2-eddyz87@gmail.com