PVF: ensure job processes are cleaned up, add tests (#2643)

Fixes a potential memory leak.

`PR_SET_PDEATHSIG` is used to terminate children when the parent dies.
Note that this is subject to a race. There seems to be a raceless
alternative [here](https://stackoverflow.com/a/42498370/6085242), but
the concern is small enough that a bit more complexity doesn't seem
worth it. Left a bit more info in the code comment.

polkadot/node/core/pvf/src/worker_interface.rs

@@ -105,9 +105,9 @@ pub async fn spawn_with_program_path(
 		gum::warn!(
 			target: LOG_TARGET,
 			%debug_id,
-			?program_path_clone,
-			?extra_args_clone,
-			?worker_dir_clone,
+			program_path = ?program_path_clone,
+			extra_args = ?extra_args_clone,
+			worker_dir = ?worker_dir_clone,
 			"error spawning worker: {}",
 			err,
 		);