Currently I have to test app and set the different fake GPS locations on real WP 8 devices. On Android I use 'My Fake Location'. Unfortunately, I didn't find the same app for WP 8. Any ideas to solve my problem? For 'black box' testing.
Testing has to be desgined during (or before) software development. Not after that.
There is good reason that on real devices there is no possibility to fake GPS from outside of that application.
To solve your problem, the app has to implement a playback mode, where it reads the locations from a csv file (or gpx), and calls a timer, e.g once a second, and creates the location programatically.
This then can be used to test specific behaviour which is not easy to achieve in real world situations. (e.g driving with 220 km/h) .
This playback feature is not visible when a specific configuration (file) is set for mass rollout of the app.