fix: NetcodeIntegrationTest preserve player prefab

[NoelStephensUnity]

This fixes an issue where the player prefab would get destroyed if you shutdown the NetworkManager assigned to the NetworkManager.Singleton (which is typically the server). This would occur with any test prefab created with NetcodeIntegrationTest.CreateNetworkObjectPrefab.

Testing and Documentation

• No documentation changes or additions were necessary.
• Includes Integration Test: PreserveIntegrationTestPrefabs

[SamuelBellomo]

It'd be great if there could be a quick netvar sync or RPC call here, just to make sure those NMs are really connected together. I'm saying this cause we had silent fails in BR tests where NMs wouldn't connect and the tests would still pass (the connection failures would be outputed as warnings, ignored by our tests)

[0xFA11]

is this still a relevant PR or should we close it? @NoelStephensUnity

[NoelStephensUnity]

is this still a relevant PR or should we close it? @NoelStephensUnity

I have been meaning to circle back to this... it still needs some work and is something that could be useful.
We can always keep the branch but close out the PR if needed.

[NoelStephensUnity]

Closing this PR as this integration testing feature should be looked into after some up-coming architectural changes.