tests: avoid zombie lockup with test-hup

The 'while kill -0' recipe can cause a livelock if the process we're waiting
to die is a normal child process. If it becomes a zombie that the shell
doesn't reap (shell bug?), it will continue to be able to accept
signals. So instead, we just wait(1).

tests/test-hup.t

@@ -15,7 +15,7 b' Do test while holding fifo open'
   > kill -HUP $P
   > ) > p
 
-  $ while kill -0 $P 2>/dev/null; do true; done
+  $ wait
   $ cat out
   0
   0