check that video saved to sdcard

Test to complete issue #104.
Check that video is saved to sdcard.

Test fails on `master` (it actually xfailed due to a bug, look at manifest.ini)
Test passes on `v1-train`.

I used `mozdevice` to get list of items in media directory. 
`self.data_layer.media_files` is not reliable and fails with `TypeError` (I think for both master and v1-train)