Exclude Unity.Rendering.HighDefinition.Tests from docs

Unity: HD RP

2023.1.X

2022.2.X

Backported to 2021.2 and 2022.2

Fixed in: 2022.2 Documentation

API in Unity.Rendering.HighDefinition.Tests is currently public.

This should be excluded from the docs.
One way to do this is to create a [Internal link].