* Add tests for applying added prefab instances to prefabs
* Fix nested prefab instances getting wiped when applying changes
Prefab update serialization only kept instance data for nested roots sitting directly under the instance root, deeper ones (under group objects) got their mappings wiped in the prefab cache scene where they can't be rebuilt.
Also only convert children to nested after applying, flagging the applied root itself made it get silently adopted when dragged into another instance later.
* Move all unit tests to Engine/Tests
* Fix Margin.EdgeSubtract
* Fix Capsule.Contains
* EnvironmentVariables.Remove if it's null
* Fixed ray trace never returning startedsolid
* Fix HistoryList.Navigate on empty list
* Fix GameObject.WorldPosition accepting NaNs
* Fix flex: initial expanding to the wrong grow/shrink
* Translation.TryConvert shouldn't throw on invalid enum strings
* Skip sound file tests on machines with no audio device
