Test that --on-job-exit functions run even if the job has already exited

If you define a function that you want to be called after a job has
exited, it should run immediately if that job has already exited.

tests/checks/jobs.fish

@@ -123,3 +123,10 @@ emit bar
 command true &
 set -l truepid $last_pid
 test $truepid != $fish_pid || echo true has same pid as fish
+
+# Job exit events work even after the job has exited!
+sleep .5
+function thud --on-job-exit $truepid
+    echo "thud called"
+end
+# CHECK: thud called