from https://flexunit.tenderapp.com/kb/getting-started/flash-builder-4-standard

FlexUnit4_in_FlashBuilderStandard_RunnerApplication.mxml

<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
               xmlns:s="library://ns.adobe.com/flex/spark" 
               xmlns:mx="library://ns.adobe.com/flex/mx"
               xmlns:flexui="flexunit.flexui.*"
               xmlns:adobe="http://www.adobe.com/2009/flexUnitUIRunner"
               minWidth="955" minHeight="600" 
               creationComplete="onCreationComplete()">
    <fx:Script>
        <![CDATA[
            import org.flexunit.listeners.UIListener;
            import org.flexunit.runner.FlexUnitCore;

            import tests.MainCase;

            private var core:FlexUnitCore;

            public function currentRunTestSuite():Array
            {
                var testsToRun:Array = new Array();
                testsToRun.push(tests.MainSuite);
                return testsToRun;
            }


            private function onCreationComplete():void
            {
                core = new FlexUnitCore();
                core.addListener( new UIListener( testRunnerBase ) );
                core.run( currentRunTestSuite() ); 
            }

        ]]>
    </fx:Script>
    <fx:Declarations>
        <!-- Place non-visual elements (e.g., services, value objects) here -->
    </fx:Declarations>
    <adobe:TestRunnerBase id="testRunnerBase" width="100%" height="100%" />
</s:Application>