Fixes FlipView unit test failures. Every failure was caused by the FlipView unit tests depending on setImmediate's behavior for handling async stuff. Since setImmediate isn't available on Chrome/Firefox/Safari, and WinJS.Utilities._setImmediate doesn't use msSetImmediate on IE10, these environments were handling the async callbacks different than IE11 with setImmediate. This change fixes the tests to not depend on setImmediate's behavior by listening to an event that fires after an item is finished rendering, and also changes a couple datasource tests to use a notification signal instead of a timeout