Repeat the process tree walk and spare processes started by signal handlers

tests/unit/preflight.rs

@@ -0,0 +1,12 @@
+use super::*;
+
+#[test]
+fn proc_children_check_accepts_existing_file() {
+    assert!(check_proc_children(Path::new(PROC_CHILDREN)).is_ok());
+}
+
+#[test]
+fn proc_children_check_rejects_missing_file() {
+    let result = check_proc_children(Path::new("/proc/thread-self/no-such-file"));
+    assert!(matches!(result, Err(SandboxError::ProcChildrenMissing)));
+}