Allow passing an optional timeout to Testability's whenStable(). If
specified, if Angular is not stable before the timeout is hit, the
done callback will be invoked with a list of pending macrotasks.
Also, allows an optional update callback, which will be invoked whenever
the set of pending macrotasks changes. If this callback returns true,
the timeout will be cancelled and the done callback will not be invoked.
If the optional parameters are not passed, whenStable() will work
as it did before, whether or not the task tracking zone spec is
available.
This change also migrates the Testability unit tests off the deprecated
AsyncTestCompleter.
PR Close#16863
Michael Giambalvo
Jason Aden