Require 'uri' in GDK::TestURL to fix NameError

What does this merge request do and why?

This fixes an error that's thrown when running support/test_url. The error looked like:

❯ ./support/test_url http://192.168.50.25:3000

/home/mark/dev/gdk/lib/gdk/test_url.rb:19:in `initialize': uninitialized constant GDK::TestURL::URI (NameError)
	from ./support/test_url:11:in `new'
	from ./support/test_url:11:in `<main>'

I tried with bundle exec just in case but it threw the same error.

As far as I can tell support/test_url hasn't been used since 12d2032f, but it would be useful for gitlab!109286 (comment 1278840650), unless support/ci/test_url is preferred?

How to set up and validate locally

Execute support/test_url

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.