Before converting most of the project to C# 7, `netstandard2.0`, etc. I want to get unit tests in shape. There's a fair bit of complication to doing so in that we need to test against:
- Different versions
- Clusters
- Master/Slave setups
- Multiple-test unfriendly tests (like SHUTDOWN commands)
- Simulate remote (read: latent) connections

This work does a few things:
- Moves all unit tests and assertions to xUnit
- Enables parallelization for tests
- Utilizes the xUnit output semantics, which means things like connection logs are on the test in VS, console output, build servers, etc. (it was a giant pool of `Console.WriteLine` mixed before)
- Cleans up the `\packages` folder completely (it wasn't used for anything but the `redis-*.exe` binaries
- Cleans up all test code formatting in general
- Re-enables `net462` in testing (was only `netcoreapp1.0` before)
- Allows overrides via `TestConfig.json`
- Updates all the RedisConfig\ scripts for easy test running
- Adds a StackExchange.Redis.Tests\README.md
- Fixes many tests

Remaining Long-term TODOs
- Look at Redis 4.0 on Windows via WSL
- Configure endpoints in a `.json` config override that's in `.gitignore`
- Find a way to handle master/slave breaks on underpowered machines better