Revise tests in test_tortls.c to not act intrusively on openssl internals

With openssl 1.1, most openssl structures will become opaque. But the new unit tests in test_tortls.c assume that they have full access to openssl internals.

I've wrapped the tests that make this assumption inside #ifndef OPENSSL_OPAQUE, so that Tor will build with openssl 1.1 again. But it would be great to see if we can port some of these tests to work with openssl 1.1, or at least disable the problematic parts on a more fine-grained basis.