test: cover tls multi-identity option mixtures

Prove that cert and key options do not have to be ordered, and that the
pfx option can be used at the same time as the cert/key option
(which was claimed to be impossible by some pre-existing documentation).

PR-URL: https://github.com/nodejs/node/pull/24374


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>