Expose megaYield() count

[BastianKusserow]

We are currently about refactoring our codebase from Combine to Concurrency and AsyncStreams.
However, we have a test that schedules some work sequentially and now tests that passed reliably with Combine are now pretty flakey.

Increasing the Task.megaYield() count from 10 to something like 20 seem to let the tests pass reliably.
I cannot really share the code here and also are not really able to construct a simple test case for the issue, but due to the mere fact that increasing the yield count solves the issue I do think thats just the nature of swift and concurrency tests.

Therefore maybe its worth exploring how to expose the yield count somehow :)