Elaborate job-y.tst test case

This commit syncs the test file with:
https://github.com/magicant/yash-rs/blob/a54b86bd943910e18916943983fa93e8738a27ac/yash-cli/tests/scripted_test/job-y.sh

tests/job-y.tst

@@ -1,6 +1,21 @@
 # job-y.tst: yash-specific test of job control
 ../checkfg || skip="true" # %REQUIRETTY%
 
+# This test case first creates a background job that immediately exits, then
+# waits for the job to finish, sending a null signal to the job to poll if the
+# job is still running. A subshell starts another job and waits for it to finish
+# to make sure the main shell process receives the SIGCHLD signal and examines
+# the latest job status. The test case checks if the job is reported as done
+# before the prompt for the next line is displayed.
+test_e 'interactive shell reports job status before prompt' -im
+echo >&2; sleep 0& while kill -0 $! 2>/dev/null; do :; done; (sleep 0& wait)
+echo done >&2; exit
+__IN__
+$ 
+[1] + Done                 sleep 0
+$ done
+__ERR__
+
 mkfifo sync
 
 # According to POSIX, a shell may, but is not required to, forget the job