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

tools/testing/selftests/bpf/progs/btf_dump_test_case_padding.c

@@ -102,12 +102,21 @@ struct zone {
 	struct zone_padding __pad__;
 };
 
+/* ----- START-EXPECTED-OUTPUT ----- */
+struct padding_wo_named_members {
+	long: 64;
+	long: 64;
+};
+
+/* ------ END-EXPECTED-OUTPUT ------ */
+
 int f(struct {
 	struct padded_implicitly _1;
 	struct padded_explicitly _2;
 	struct padded_a_lot _3;
 	struct padded_cache_line _4;
 	struct zone _5;
+	struct padding_wo_named_members _6;
 } *_)
 {
 	return 0;