Specify certfile to verify SSL certs against in tests

#162 added a TLS Start test but didn't assert verified certs because the `tls_options` hadn't been wired up yet. #161 changed this, but hadn't been tested against the additional test in #162 before getting merged, resulting in `master` failing.

It's easy enough to disable certificate verification, but it's not necessary. Instead, we can also set the `:ca_file` TLS options to verify against.

cc @jch @schaary @sonOfRa @tarcieri 

Yeah, [considered this](https://github.com/ruby-ldap/ruby-net-ldap/pull/166#discussion_r21066025), though that creates more friction for running tests locally, or at least running the integration tests locally.

In any case, I don't think we should block this fix on figuring out generated CA certs. That can come in a followup PR. Thoughts?