Write tests for incremental upgrades

Our test suite should contain tests for this functionnality.

A cheap solution that would cover 90% of this is the following: we create a thrid channel (beyond stable and alpha) that publishes a UDF (that we have to publish for each new version) that points to a simple IUK that simple adds some file to the Tails filesystem, deletes some others, and adds something in the live/ sub-dir, and then we install it and verify that these modifications have been applied.

Feature Branch: test/6309-incremental-upgrades

Parent Task: #6298 (closed)

Related issues

• Blocked by #5922 (closed)

Original created by @bertagaz on 6309 (Redmine)