Make colorize test pass with NO_COLOR env set (#994)

Author: tompng

test/irb/test_input_method.rb

@@ -35,6 +35,7 @@ def test_initialization
     end
 
     def test_colorize
+      IRB.conf[:USE_COLORIZE] = true
       original_colorable = IRB::Color.method(:colorable?)
       IRB::Color.instance_eval { undef :colorable? }
       IRB::Color.define_singleton_method(:colorable?) { true }