selftests/bpf: Refactor stacktrace_map case with skeleton

The loading method of the stacktrace_map test case looks too outdated,
refactor it with skeleton, and we can use global variable feature in
the next patch.

Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250925175030.1615837-2-chen.dylane@linux.dev