test: rename agent1-pfx.pem to agent1.pfx

PFX is not PEM, its binary DER. Use the same .pfx extension as
test/fixtures/test_cert.pfx does.

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>