Update uis-traffic-manager-mtls module with Boilerplate and Tests

Description

This module needs Terraform tests to ensure any changes can be validated. To facilitate this, we first need to migrate to the standard terraform-module-boilerplate, then write the tests. The tests should be run automatically.

Further details

Task list

• Migrate to Terraform Module Boilerplate
• Write unit tests to cover all functionality
• Add gcp-alert-snooze to terraform-test-cleanup CI

Acceptance criteria

• Module should be on standard boilerplate
• All functionality should be covered
• terraform-test-cleanup is configured for gcp-alert-snooze

Links/references

• Guidance on testing Terraform modules
• How to implement a Terraform testing pipeline for shared modules