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/common/src/execute.rs

@@ -96,4 +96,6 @@ pub enum JobError {
 	CouldNotSpawnThread(String),
 	#[error("An error occurred in the CPU time monitor thread: {0}")]
 	CpuTimeMonitorThread(String),
+	#[error("Could not set pdeathsig: {0}")]
+	CouldNotSetPdeathsig(String),
 }