Update testing docs to use MockFederation.createContext()

The MockContext class is no longer exported from @fedify/testing.
Updated all examples in the testing documentation to use
MockFederation.createContext() instead of directly instantiating
MockContext.

Also changed forEach to for-of loops to avoid implicit any type
errors in Twoslash type checking.

https://github.com/fedify-dev/fedify/issues/468