@drice-buf discovered that simply setting supports_tls: true in the config, and doing nother else to support TLS in a server under test, would not cause any tests to fail (!!!).

The issue is that the server was basically ignoring the use_tls field, and thus sending back an absent/empty certificate. The runner would simply forward that certificate to the client. But the client requests don't have a separate flag indicating whether or not to use TLS: they rely on the certificate being present/non-empty to enable it. So this would effectively forward the empty cert to the client, which would simply not use TLS upon seeing that the cert was absent/empty. And all of the tests requests would succeed because neither client nor server were using TLS. (If the client were using TLS but not the server, or vice versa, then every HTTP call would have failed.)

The solution is pretty simple: have the runner actually verify that the server provides a certificate when use_tls is true.

Resolves #830.