Add tests for newly deployed guardian workflows

### TL;DR

https://github.com/abcxyz/guardian/issues/537 was resolved but no tests were added to prevent it from happening again in the future. Guardian should have tests that verify that [fresh Terraform actuation workflows](https://github.com/abcxyz/guardian/issues/537) succeed.

Look at the Terraform actuation workflows referenced in issue #537. Find the existing test suite for guardian workflows, likely in a test directory. Write tests that verify new Terraform workflows deploy successfully. Check for any integration test patterns or CI configurations that run workflow tests.

索引モデルが issue の本文から書いたものです。