Provide documentation about unit testing

[ianbotsf]

Describe the issue

Users often have questions about unit testing with the SDK, specifically around mocking (which can be tricky for lambda DSL blocks) and extension methods. Write some documentation describing some recipes and best practices. This new documentation should probably be part of the Dev Guide but alternate options should be considered (e.g., blog post, in-repo Markdown, etc.).

Links

(n/a, new page[s] required)