tests/job-ids: Wait for job to die

It was possible to start the new job and execute `jobs` again before
the job died (or we noticed it did), so the test would fail.

To properly test, we need to ensure the job has been removed. `wait`
should do it.

tests/checks/job-ids.fish

@@ -33,6 +33,8 @@ jobs
 
 status job-control interactive
 command kill -9 $tokill[2]
+# Wait for the job to die - the signal needs to be delivered.
+wait $tokill[2] 2>/dev/null
 set -e tokill[2]
 status job-control full
 sleep 400 &