Fixes a CI failure

What does this MR do?

In one of the API tests, gon.relative_url_root was set to http://host.invalid which is not a relative root. This breaks the tests.

This MR replaces that route with an actual relative one which gets the tests passing again.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Closes #11315 (closed)