Write automated tests for Additional Software GUI

Originally created by @Anonymous on #14596 (Redmine)

This needs to happen before March 25th 2018 (B3)

We need:

1. fixing the “all persistence configuration files have safe access rights” step: live-additional-software.conf now has 644 permissions (on purpose), which prevents any scenario that uses persistence to run, so it was temporarily disabled in 19c6e71f
2. fixing the Synaptic scenario, that’s broken since b34e8a51
3. a scenario (described in #14572 (comment 34635) and in the offline ASP blueprint) to test the case where an ASP online package upgrade failed, and how if next boot is offline ASP do cope with it; this will validate the work that’s been merged already and shipped in Tails 3.6
4. -tests for the main scenarios described on https://tails.boum.org/blueprint/additional_software_packages/gui/-

Feature Branch: test/14596-automated-tests-for-ASP-gui

Parent Task: #14568 (closed)

Related issues

• Related to #15799 (closed)
• Blocked by #14576 (closed)