bpf: refactor bpf_test_run() to separate own failures and test program result

After commit f42ee093 ("bpf/test_run: support cgroup local
storage") the bpf_test_run() function may fail with -ENOMEM, if
it's not possible to allocate memory for a cgroup local storage.

This error shouldn't be mixed with the return value of the testing
program. Let's add an additional argument with a pointer where to
store the testing program's result; and make bpf_test_run()
return either 0 or -ENOMEM.

Fixes: f42ee093

 ("bpf/test_run: support cgroup local storage")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Roman Gushchin <guro@fb.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>