tests/param-p: Reduce test of special parameter !

The previous test assumed a particular process tree topology that is not
implemented by all shells. Since the process tree structure is
implementation detail, we can rely only on the wait status of the
signaled process.

tests/param-p.tst

@@ -451,23 +451,11 @@ kill $$
 echo not reached
 __IN__
 
-test_oE 'special parameter !'
-mkfifo fifo
-echo foo | {
-    trap 'echo trapped; exit 0' USR1
-    cat
-    cat fifo
-}&
-exec 3>fifo
-echo bar >&3
-kill -s USR1 $! # should kill the last process of the background pipeline
-exec 3>&-
+test_O -e USR1 'special parameter !'
+while kill -s 0 $$; do sleep 1; done &
+kill -s USR1 $!
 wait $!
 __IN__
-foo
-bar
-trapped
-__OUT__
 
 # Special parameter 0 is tested in sh-p.tst
 #test_oE 'special parameter 0'