Add test covering multiple prometheus alerts in single payload

What does this MR do?

Adds a spec to cover a previously untested case, where we receive multiple alerts from Prometheus in a single webhook.

This conforms to the example Prometheus docs here: https://prometheus.io/docs/alerting/configuration/#webhook_config.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content