Add a few test that validate BPF verifier's lazy approach to validating
global subprogs.

We check that global subprogs that are called transitively through
another global subprog is validated.

We also check that invalid global subprog is not validated, if it's not
called from the main program.

And we also check that main program is always validated first, before
any of the subprogs.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20231124035937.403208-4-andrii@kernel.org