Tweak performance tests to be friendlier to root tile throttling (Google P3DT)

[csciguy8]

Changes to let the performance tests hit the HTTP 429 code (too many requests) a little less.

1) Change core test logic to not clear disk cache on every run. Let individual tests configure this 2) After a performance test ran, we would let the editor scene load the same scene as a nicety (SetSuspendUpdate(false)). This is essentially a throw away scene load if you don't care about it. Don't do this either. 3) For Google Performance tests, let most tests just do a single pass (cold cache).

Also adds more profiling events during Unreal model loading (Cesium::LoadPrimitive event). This event was recently discovered as a bottleneck for certain performance tests (warm cache).

Before (Unreal Insights capture)

After

[csciguy8]

Merging this without review...

Reasons:

• It works well. I've already used this branch to generate performance reports for other PRs
• It's not user facing. Just a few developers run these (me mostly).
• It's ~2 months old and not worth disrupting others to review it.