Edit the smoke test of the Google CAS issuer so that it properly tests the CAS issuer

In #5, I decided to only smoke test cert-manager itself as opposed to testing both cert-manager and google-cas-issuer.

Today, I discovered that the testrunner that we use for the smoke tests has a "GCP runtime config" support built-in. [Google Cloud Runtime Config](https://cloud.google.com/sdk/gcloud/reference/beta/runtime-config/configs) could allow us to create the necessary GCP serviceaccount and bindings required for the CAS issuer smoke test.

检查现有的 smoke test 设置以及 #5 中记录的决策，然后检查 testrunner 对 GCP 运行时配置的支持情况。当 smoke test 使用所需的 GCP 服务账号和 bindings，同时执行 google-cas-issuer 以及 cert-manager 时，工作即告完成。

由索引模型根据 Issue 内容生成。