test_dns.rb: no server access

* test/resolv/test_dns.rb (test_invalid_byte_comment): get rid of
  actual server access.  [Bug #9273]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44318 b2dd03c8-39d4-4d8f-98ff-823fe69b080e