Adds a couple of integration tests.

[RogerSep]

Before adding a few more, I'd like to get some feedback on these to get a grasp of what you're after. If you listed the kind of tests that you want that'd be great.

Also, I just went for the fastest I could write them; as I mentioned in #77 I had some written in my application and I'm using xunit there. If you'd like to keep nunit or have the tests in a different project structure let me know. I made it its own project as these take quite a bit to run but it's your call.

Regards

[RogerSep]

BTW, I don't know why this happens but I'm getting an error when trying to run your tests:

System.TypeInitializationException : The type initializer for 'RocksDbSharp.Native' threw an exception.
  ----> NativeImport.NativeLoadException : Unable to locate rocksdb native library, either install it, or use RocksDbNative nuget package

Have you ran into this?

[LGouellec]

BTW, I don't know why this happens but I'm getting an error when trying to run your tests:

System.TypeInitializationException : The type initializer for 'RocksDbSharp.Native' threw an exception.

  ----> NativeImport.NativeLoadException : Unable to locate rocksdb native library, either install it, or use RocksDbNative nuget package

Have you ran into this?

If you run on Linux (maybe in macOS too), you need to install some dependencies for RocksDb support.

You can check my CI script to install these :

https://github.com/LGouellec/kafka-streams-dotnet/blob/develop/.circleci/config.yml

[LGouellec]

I'm starting your PR. First thanks for your contribution.

Some feedbacks :

• I prefer keeping NUnit for maintenance even if Your work with Xunit is great. If you can switch to NUnit with same structure will be wonderful.

• Great idea to create a dedicated project for integration. All right ! As I triggered this project for PR, and merge to master for example.

So just switch to NUnit and you can continue integration test with join, aggragate, some state full tests.

Perfect !! 😀🚀