Generate or build certs during tests

In the other Cloud SQL Connector repos (Python, Java, Go) we generate/build certs for the tests as part of the tests themselves or test infra.

Node currently has certs that were generated once and are [saved (hardcoded) for tests](https://github.com/GoogleCloudPlatform/cloud-sql-nodejs-connector/blob/4e3f31bff33cc6de8a21ff61ca1c96703bb66dd5/test/fixtures/certs.ts). This is not scalable for different cert types (CAS-based CAS for example) and requires generating new certs and updating the hardcoded values.

Instead node should generate/build certs within its test infra as the other Connector libraries do.