selftests/bpf: Add checks for X__elf_bytes() skeleton helper

This patch adds two checks for the X__elf_bytes BPF skeleton helper
method. The first asserts that the pointer returned from the helper
method is valid, the second asserts that the provided size pointer is
set.

Signed-off-by: Matt Smith <alastorze@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210901194439.3853238-4-alastorze@fb.com