Make it possible to acccess TestContext.Renderer without putting TestServiceProvider into provider mode

[egil]

The ITestRenderer is being pulled from the TestServiceProvider lazily the first time it is requested. However, this is not really a good design decision, since it a property should not have side effects like this. Its surprising to the user.

However, in the interest of not breaking the API for v1, I think this has to stay as is for now.

[linkdotnet]

That would be obsolete if we do #1018

[egil]

Perhaps. We need the renderer in relation to MarkupMatches and a few other places. Will need to investigate.