add test for Regexp.new("\0").

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@13814 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

bootstraptest/test_knownbug.rb

@@ -61,3 +61,7 @@ assert_equal 'ok', %q{
 assert_equal 'ok', %q{
   0**-1 == 0 ? :ng : :ok
 }
+
+assert_equal '(?-mix:\000)', %q{
+  Regexp.new("\0")
+}