Stopped SpriteAtlas tests from failing on Yamato

[unitystevem]

Description

If the SpriteAtlas test assets aren't properly created/saved for some reason (as is the case on Yamato), skip the tests that use those assets in order to prevent test failures that are outside our control.

Checklist

Before review:

• [ ] Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
• [X] Tests added/changed, if applicable.
  • Changed functional tests SpriteAtlas_Not_Empty_Is_Not_Reported and SpriteAtlas_Empty_Is_Reported, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
• [ ] Docs for new/changed API's.
  • Code is annotated using Xmldoc syntax.

[mtrive]

Closing for the time being since Emeline is investigating.