Resolve "Extend the reach of GDK_DEBUG to enable debugging in CI"

What does this Merge Request do and why?

This MR aims to make it easier to enable debugging in the context of CI.

Merge Request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise Issue to follow-up.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

Closes #1261 (closed)