Due to a bug in OpenSSL 1.1.0h[1] (it's only in this specific version;
it was introduced just before the release and is already fixed in their
stable branch), the callback set by SSLContext#session_new_cb= does not
get called for clients, making net/http and net/ftp not attempt session
resumption.

Let's disable the affected test cases for now. Another option would be
to fallback to using SSLSocket#session as we did before r64234. But
since only a single version is affected and hopefully a new stable
version containing the fix will be released in near future, I chose not
to add such workaround code to lib/.

[1] https://github.com/openssl/openssl/pull/5967

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64252 b2dd03c8-39d4-4d8f-98ff-823fe69b080e