add a test for [ruby-dev:33807]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@15503 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_knownbug.rb

@@ -56,3 +56,6 @@ assert_normal_exit %q{
 assert_normal_exit %q{
   Encoding.compatible?("",0)
 }
+assert_normal_exit %q{
+  "".center(1, "\x80".force_encoding("utf-8"))
+}, '[ruby-dev:33807]'