* Move all the RunAsync logic to the TestRuntime class, instead of having some
in TestRuntime and some in AppDelegate.
* Create a unified Task-based implementation for all platforms, optionally showing
a UI on the platforms where we can do that.
* Remove all the overloads that took a DateTime timeout, and instead only use a
TimeSpan timeout. This simplified some of the code.
* The new Task-based implementation will capture any exceptions (and rethrow most
of them) from the tasks we're waiting for, so no need to do that in each RunAsync
caller. This simplifies the testing code a lot for some tests.
* Add a new TryRunAsync method that will return (instead of rethrowing) any exceptions.
This simplifies some of the testing code (which verifies the correct exception,
or ignores the test in case of some exceptions).
* The new Task-based implementation will bubble up any NUnit exceptions, which
means that the tasks we're waiting for can call NUnit's Assert and the right thing
happens (in particular Assert.Ignore will actually ignore the test).
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