Create mosaic-testing module

[EpicDima]

Fixes #4

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• CHANGELOG.md's "Unreleased" section has been updated, if applicable.

[JakeWharton]

I really want to land this, but I want to play around with the API locally first. I just haven't had a lot of free time lately. I will try to get to it some day this week, hopefully tomorrow.

I think we can avoid exposing internal API for this by making the base testing API a little more generic. This would allow the normal testing API to still provide consuming of the rendered output, but the commonTest of the runtime could still consume the internal nodes.

[JakeWharton]

Just a rebase so I can start looking locally. I'm off for two weeks after tomorrow, so hoping to get some movement here.

At the very least, the test API needs changed to track the changes in Redwood and #507 where we get rid of the channel and capture changes after a call to sendFrame.

[JakeWharton]

Note: still very broken, but closer. I want to pull out a few more changes ahead of this. We're close!