test: clarify confusion over "client" in comment

Fix perplexing comment. It's not that TLS "clients" don't support
'secureConnect', it's that client sockets created with `new TLSSocket`
(as opposed to `tls.connect()`) don't support that event.

PR-URL: https://github.com/nodejs/node/pull/25508


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>