Code review comment for lp://qastaging/~nick-dedekind/unity/lp1062107.preview-pre-caching

@andyrock: As stated in the description, the supplied test is only meant to check the preview pre-caching sequence (pretty much only the PreviewPreCacher::ContinuePreCaching function).

Getting a lens view into the test harness to test the pre-cacher interaction is somewhat controversial and not suited to unit testing at the moment (Lens Dee related code is "too tight" for testability).
I am hoping to get some refactoring into the lenses soon so they can be properly tested. Once this is done, we can revisit this.