Unit testing activities in Elsa 3

It is common to have custom activities in Elsa. For these activities the developers may want to write unit tests.

Currently, we have a way to write integration tests (executing workflows as code), there are a few examples in code of how to do that. However, I was not able to find examples of writing unit tests for an activity. This issue is about providing examples of how to do that, either in code or in the documentation.