What

Adding a missing scenario test for "gpc.ack" endpoint

Why

Testing for "gpc.ack" interaction was missed in the integration tests, so this PR adds coverage for it.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Internal change (non-breaking change with no effect on the functionality affecting end users)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the Changelog with details of my change in the UNRELEASED section if this change will affect end users
• A corresponding change has been made to the Mapping Documentation repository