Add docs about DI and mocking for tests

[craigfowler]

One thing that comes up when using specs is how to test them and how to test things which consume them. What this is really about is the consumption of specification objects. Whether they're consumed as interfaces or concrete classes, and the consequences of this for testing.

There are a few ways:

• Write one-line interfaces for each spec
  • Then the dev just mocks the interface as usual
• Use virtual members which can be overridden
  • No need for an extra interface, and the dev can mock an object with virtual members
• Write and use a service locator
  • Anti-pattern but it would work to provide specs as interfaces

These techniques should be documented.

[craigfowler]

I could do slightly better than service locator. I could provide a fluent factory for this, like:

specFactory.GetFor<MyEntity>().UsingImpl<MyEntitySpec>();

That could return a specification interface in its merits signature. The default/prod impl would get an instance of that spec from DI and return it. It could even pass ctor patterns to the spec impl if applicable.

In a test environment though, that factory could/would return a mock spec instead.