Test webhook payload in issue hook specs

What does this MR do and why?

This change verifies relevant parts of the webhook payload in our existing tests of webhooks for issue services.

It's in preparation for us changing the way that webhooks are executed slightly in !100104 (closed).

Related: #326528 (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.

Related to #326528 (closed)