Automate test of incremental upgrades

Originally created by @sajolida on #10480 (Redmine)

While working on #10394 (closed) I noticed that there is not ticket for the automation of incremental upgrades, neither on https://tails.boum.org/contribute/release_process/test/ neither in Redmine. Is that on purpose?

Edited May 15, 2020 by sajolida

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