Add default failure target if one step times out.

tests/su/01/run_su.exp

@@ -1,6 +1,7 @@
 #!/usr/bin/expect
 
 set timeout 2
+expect_after default {puts "\nFAIL"; exit 1}
 
 if {$argc != 3} {
 	puts "usage: run_su.exp <user> <password> <prompt>"