Add 'gdk.test' to documentation and other improvements

What does this Merge Request do?

Closes: #918 (closed).

Makes some other improvements while I'm here.

Merge Request checklist

• Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• gdk doctor test added, if needed.