Extend tests in gitlab-org/build/CNG to cover the database automation functionality

Context

This is part of Implement auto deployments for the Container Re... (&7998 - closed) and is identified as task B in the overall plan.

Problem

We manually create version bump MRs on the gitlab-org/build/CNG project whenever we release a new version of the registry (sample). However, in #673 (closed), we automated the creation of the version bump MRs.

However, we don't have any tests to cover this new automated step.

Solution

• Extend tests in gitlab-org/build/CNG to cover the database automation functionality;
• Automatically trigger test suite for version bump MRs.