Add How-to documentation for test IConfiguration scenarios

As per feedback at https://github.com/corvus-dotnet/Corvus.Configuration/pull/79#pullrequestreview-376160461 -

> not _all_ common scenarios are covered - there's no mention of the code that allows you to set up `IConfiguration` using a test file.

Start with the feedback on pull request 79 and inspect the existing how-to documentation for test IConfiguration scenarios. Add the missing explanation of the code that sets up IConfiguration using a test file, with the common scenario covered when the documentation is complete.

Written by the indexing model from the issue text.