* exec: Spawn processes as direct children
Spawn processes as children rather than grandchildren.
This way, spawned processes may track Hyprland's state
by watching their parent, either directly or indirectly
(e.g., Linux's `PR_SET_PDEATH_SIG`).
Fixes#11728
* tests/exec: Add the test on process spawning
Add a test that ensures that:
- A spawned process remains a direct child of Hyprland;
- Upon termination, the process does not become a zombie.
