Update testing guide for Model

[ptrgags]

this PR goes through the TestingGuide and updates details that were assuming the old Model implementation, and updates links to glTF models that have been reorganized.

@j9liu could you review when you get a chance?

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.
• ❔ Unit tests were not updated.
  • Make sure you've updated tests to reflect your changes, added tests for any new code, and ran the code coverage tool.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[j9liu]

@ptrgags looks good, I'll just wait for Travis to pass and merge. Thanks!