To make this happen, we need to take as argument a TLSDialer rather than
a TLSHandshaker. Then, we need to arrange the code so that we always
enforce a timeout for both TCP and TLS connections.
Because a TLSDialer can be constructed with a custom TLSConfig, we cover
also the case where the users wants to provide such a config.
While there, make sure we have better unit tests of the HTTP code.
See https://github.com/ooni/probe/issues/1591