Integration test changes

[ElectroJr]

#19370, #19496, space-wizards/RobustToolbox/pull/4283 have made some changes to content integration tests. In particular:

• The PairTracker & Pair classes have been merged into one TestPair class. The old PairTracker.Pair field is now unnecessary.
• Several PoolManager functions are now obsoleted and will be moved over to the TestPair class.
• The client/server instances have some new fields to make writing tests slightly easier:
• Some common resolves now have fields. E.g. server.EntMan can be used instead of server.ResolveDependency<IEntityManager>()
• Entity systems can be resolved via server.System<T>()
• There are also transform & metadata component helpers.