Add test suite workaround for #8010.
Simplify interaction with Synaptic.

... by using more images. Relying on keyboard shortcuts requires that
the right window has focus, and that behaviour changed at least with
the migration to GNOME Shell (as part of the Debian Jessie migration).
liveusb-creator depends on the old udisks, so install it.
Make removable devices user writable.

This fixes #8273, but I'm not sure whether it's a proper solution or
just a workaround.
Merge remote-tracking branch 'origin/feature/jessie' into feature/jessie