[TLS, V3] Have tests check for certs when spinning up and generate new ones if needed

[mgattozzi]

This is a follow up to #26246.

We generated a hard coded cert for the repo for all of our testing and to use TLS by default for our tests. However, this is expected to expire in 1 year:

$ openssl x509 -in /tmp/pem -text -noout
        Validity
            Not Before: Apr  9 22:27:29 2025 GMT
            Not After : Apr  9 22:27:29 2026 GMT

We need to do the following:

1. Remove the hard coded certs
2. Have the paths for those certs added to .gitignore
3. Have tests generate those certs if they do not exist and they have not expired yet

With this we can avoid the ticking time bomb and anyone should be able to run the test suite without any problems.