Add new test cases for GCP

What does this MR do?

Add new tests, migrated from the previous test project at gitlab-org/security-products/tests/secrets!55 (closed). I've split up the test cases from that MR into the various provider-specific MRs that introduce or update detection rules.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab/-/issues/375201+

Does this MR meet the acceptance criteria?

• Changelog entry added
  • Not adding one
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer

^ All are not applicable