[QoL] Allow RenderTester to recover and continue from missing AnyScreen-producing child workflows

[bencochran]

AnyScreen is opaque and we can easily construct one to return from RenderTester's test RenderContext in situations where a child workflow that renders an AnyScreen hasn't been expected (and thus we've been given no value to return) but one is rendered. This still fails the tests, but allows it to continue rather than crashing.

One thing this does is add a dependency from WorkflowTesting to WorkflowUI. Are we okay with that?

If we like this overall approach, we could also consider adding paths for other types we're able to construct like String, Int, etc. (even though I think it's actually pretty rare that people actually use these types of types as renderings in practice).

(I also tried to make the message in the case were we do crash a little less terse (explaining a bit of why we have to crash))

Checklist

• [X] Unit Tests
• [X] ~UI Tests~ (N/A)
• [X] ~Snapshot Tests (iOS only)~ (N/A)
• [X] ~I have made corresponding changes to the documentation~ (N/A)