selftests/bpf: Update btf_dump case for conflicting names

Update btf_dump case for conflicting names caused by forward declaration.

Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220301053250.1464204-3-xukuohai@huawei.com