Undef on_parallel_worker? before redefine.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@36276 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/test/unit/parallel.rb

@@ -163,6 +163,7 @@ if $0 == __FILE__
   module Test
     module Unit
       class TestCase < MiniTest::Unit::TestCase
+        undef on_parallel_worker?
         def on_parallel_worker?
           true
         end