shelltest: use a more magical magic number for success.

There may be some odd situation where a shell returns 0 despite not executing
the script correctly; now 0 is an error condition.

t/shelltest.do

@@ -2,7 +2,7 @@ set +e
 ( . ./shelltest.od )
 RV=$?
 case $RV in
-	0) exit 0 ;;
+	40) exit 0 ;;
 	42) exit 0 ;;
 	*) exit 1 ;;
 esac

t/shelltest.od

@@ -365,4 +365,5 @@ set x y z
 . ./dotparams.od a b || fail 115
 [ "$1-$2-$3" = "x-y-z" ] || fail 116
 
+[ -n "$FAIL" ] || exit 40
 exit $FAIL