Add tests for multithreading.

[StevenTCramer]

Pete and I just had a chat with Pete about

https://twitter.com/MrPeterLMorris/status/1629086636658114560

and how it could effect Blazor-State server side.

Pete tests Fluxor with the following.

https://github.com/mrpmorris/Fluxor/blob/14a8a42bb0dc610f4754c555c9b9830a34700ac3/Source/Tests/Fluxor.UnitTests/StoreTests/ThreadingTests/DispatchTests/DispatchTests.cs

We should implement this test confirm it causes a failure and if so fix it by adding locking to make it thread safe.