[Dmitry Gozman (dgozman)]

Summary

• Expect(callback).ToPassAsync() retries a Func<Task> or an Action until it passes; options are Timeout (30 seconds by default, SetDefaultExpectToPassTimeout changes it) and Intervals.
• The NUnit base classes run each attempt in an isolated NUnit context, because NUnit fails a test for an assertion failure that was caught.
• Every attempt runs on the thread pool, also when a handler calls ToPassAsync on the dispatcher thread.
• Stacked on the roll in chore(driver): roll to 1.66.0-next (8534271f0) #3464, which brings the generated API; the first commit goes away once that merges.

Fixes #2314