If you are developing a Windows Phone 8 App targeting specially the Windows Phone 8 and its features , you will be able to test it on the Windows Phone 8 Emulators .
Is it possible to develop and test Windows Phone 8 Apps with WP7 device ?
If you already have the Windows Phone 7.5 or Windows Phone 7.8 , you won’t be able to test the app targeting Windows Phone 8 on Windows Phone 7.8 device .
But if the Windows Phone app targets Windows phone 7.8 , it should run fine in Windows Phone 8 devices.