Add tests for /lib/gdk/config_type/string.rb

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What does this merge request do and why?

I added tests for /lib/gdk/config_type/string.rb.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Run bundle exec rspec ./spec/lib/gdk/config_type/string_spec.rb.
2. The tests should pass.

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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 an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.

See also: #371
Related to: #2253