test: add instructions for testing against GDK

Description

This MR adds instructions and associated env variables to be able to point the integration tests to a GDK instance

There are no changes to any pipelines, this change is just to enable another option for local testing so developers do not have to use gitlab.com.

Related Issues

#1161 (closed)

How has this been tested?

Against my local GDK

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap