Xfail ftu tests bug 850313

https://bugzilla.mozilla.org/show_bug.cgi?id=850313 strikes again
 [B2G][Browser] Cannot browse via the cellular network after manual flash Unagi

I had to log another bug 
https://bugzilla.mozilla.org/show_bug.cgi?id=893816 Cannot connect to Cellular Data

v1-train -> Merge.
master -> Manually tested only (cannot run tests due to js bug).