Ensure exceptions thrown by components during async renders are propagated to test thread

[egil]

When a component re-renders asynchronously, any exceptions thrown should be propagated to the test thread. The related question, #318, shows a case where this did not happen, making it hard to diagnose the issue with the test.

[egil]

After trying out a few ways to achieve this, the best option I've come across is to have the WaitFor* methods throw the any unhandled exceptions caught by the renderer. This will allow users who have async code to capture the exceptions. Sync code will simply surface the exception normally, so I think this is a pretty decent solution.