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 

Alternatively, we could generate a CA file if none is found instead of sticking one in test fixtures. Unsure if that'd be any better, though.