selftests/bpf: Add child argument to spawn_child function

Adding child argument to spawn_child function to allow
to create multiple children in following change.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240905115124.1503998-3-jolsa@kernel.org