Add a testing helper package

[erdtsieck]

For testing projections or just store operations.

I will build the initial package myself and add examples to the docs

[jeremydmiller]

And automatically moving this to a discussion please.

[erdtsieck]

I created a Marten.TestHelpers package and added samples for testing one offs, async projections and async listeners: #2903

This is an initial set-up of course, feedback is welcome