add test for [ruby-dev:32604].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@14231 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_knownbug.rb

@@ -169,3 +169,11 @@ assert_equal '[nil, []]', %q{
   r.inspect             
 }, '[ruby-dev:32567]'
 
+assert_normal_exit %q{
+  require 'stringio'
+  s = ""
+  10000.times {
+    s = "a".instance_eval { @a = StringIO.new(s); self }
+  }
+  Thread.new { GC.start }.join
+}, '[ruby-dev:32604]'