selftests/bpf: Assert link info uprobe_multi count & path_size if unset

Add assertions in `bpf_link_info.uprobe_multi` test to verify that
`count` and `path_size` fields are correctly populated when the fields
are unset.

This tests a previous bug where the `path_size` field was not populated
when `path` and `path_size` were unset.

Signed-off-by: Tyrone Wu <wudevelops@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20241011000803.681190-2-wudevelops@gmail.com