Make sure testMovie is called as a function and Save the output before closing:
var folderPath = path/to/folder/file.fla
if (fl.fileExists(folderPath))
{
fl.openDocument(folderPath);
fl.getDocumentDOM().testMovie();
fl.outputPanel.save("file:///C:/testJSFL/output.txt");
fl.getDocumentDOM().close(false);
}