update test_silo_certificates

Theses tests are intended to validate what happens when we use the wrong certificate to connect to a silo

https://github.com/oxidecomputer/omicron/blob/ea2f4d2de939ea47780f3bf3c1bf7d2bd6007964/nexus/tests/integration_tests/certificates.rs#L588-L627

As @jmpesp notes:

I'm not sure this check is right: it looks like this part of the test is meant to check that a client can't connect using the wrong certificate, and this check is asserting it won't trust a self-signed certificate. That's not quite the same thing. I think we need either add_root_certificate or danger_accept_invalid_certs in order to test that the client accepts the self-signed cert but fails due to the incorrect certificate being used.