selftests/bpf: Add split BTF basic test

Add selftest validating ability to programmatically generate and then dump
split BTF.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20201105043402.2530976-6-andrii@kernel.org