Key space isolation tests

Added the tests I wrote for the extension in my company's product code base. After converting from MsTest to NUnit.

The good stuff: These 248 test verifies the wrapper classes thoroughly.

The maybe not so good stuff?:

* Added [Moq](https://www.nuget.org/packages/moq/) to `StackExchange.Redis.Tests`.
* Added `[assembly:InternalsVisibleTo("StackExchange.Redis.Tests")]` to `StackExchange.Redis`.