test_ssl_server.rb: assert_self_signed_cert
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_ssl_server.rb: assert_self_signed_cert
extract common assertion.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54957 b2dd03c8-39d4-4d8f-98ff-823fe69b080e