feat: networkscenemanager interface override (backport)

[NoelStephensUnity]

Summary

This removes all unit test checks within NetworkSceneManager in order to allow for better integration testing. This version intercepts the SceneManager's Load and Unload via the ISceneManagerHandler implementation that is overridden during unit testing in order to prevent clients from actually loading scenes when they don't need to and it provides the ability to allow clients to load scenes during unit/integration tests.

Additional Changes

• Made minor tweaks to SceneEventTests
• Adjusted some Transport tests to destroy the GameObject and not just the component
• Adjusted the BaseMultiInstanceTest Coroutine destruction to destroy the GameObject and not just the component.

MTT-1672

This is a backport of #1405 into the develop branch (from experimental-develop)

Testing and Documentation

• No tests have been added (all existing tests validate the changes made)
• No documentation changes or additions were necessary.