Extract in-memory portion of TestConfigurationSource

[TheTribe]

Currently, TestConfigurationSource is the only concrete implementation of IConfigurationSource that supports an in-memory XML object as its physical config source. TestConfigurationSource still makes sense, but there should be an abstract InMemoryConfigurationSource that supports this paradigm also, allowing TestConfigurationSource to be extremely simple, and allowing other types in domain-specific contexts to use obtained XML documents as the final basis for config section retrieval.