selftests/bpf: Test multi-split BTF

Extend split BTF test to cover case where we create split BTF on top of
existing split BTF and add info to it; ensure that such BTF can be
created and handled by searching within it, dumping/comparing to expected.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250519165935.261614-3-alan.maguire@oracle.com