selftests/bpf: Add test for the packed enum member in struct/union

Add a simple test to the existing selftest program in order to make
sure that a packed enum member in struct unexceeds the struct_size.

Signed-off-by: Yoshiki Komachi <komachi.yoshiki@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1583825550-18606-3-git-send-email-komachi.yoshiki@gmail.com