Rename service to integration in Slack/Mattermost tests

What does this MR do and why?

This contributes to our effort to rename services to integrations in the codebase (&2504 (closed)).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.