fix(test): declare vars in .tftest.hcl to remove Terraform deprecation warnings

mentioned in merge request gcp-container-image-mirror!39 (merged)

mentioned in merge request ucam-gcp-secret!6 (merged)

mentioned in merge request gcp-site-monitoring!48 (merged)

mentioned in merge request ucam-faas-long!26 (merged)

mentioned in merge request gcp-cloud-run-app!122 (merged)

mentioned in merge request gcp-notification-to-gitlab-issue!24 (merged)

mentioned in merge request gcp-secret-manager!42 (merged)

mentioned in merge request ucam-faas!48 (merged)

mentioned in merge request gke-cluster!85 (merged)

mentioned in merge request gcp-function!34 (merged)

mentioned in merge request gcp-cloud-run-function!45 (merged)

mentioned in merge request gcp-scheduled-script!45 (merged)

mentioned in merge request gcp-alert-snooze!32 (merged)

mentioned in merge request ucam-observe-remote-server!28 (merged)

mentioned in merge request gcp-pubsub-to-ms-teams!31 (merged)

mentioned in merge request gcp-monitoring!3 (merged)