Let pretty_inspect work once at first

For older pp.rb which did not need io/console, and dealing with
`LoadError`.

tool/lib/core_assertions.rb

@@ -33,7 +33,7 @@ module Test
     module CoreAssertions
       require_relative 'envutil'
       require 'pp'
-      require 'io/console'
+      nil.pretty_inspect
 
       def mu_pp(obj) #:nodoc:
         obj.pretty_inspect.chomp