selftests/bpf: validate generic bpf_object and subskel APIs work together

Add a new subtest validating that bpf_object loaded and initialized
through generic APIs is still interoperable with BPF subskeleton,
including initialization and reading of global variables.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20241023043908.3834423-4-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>