Testing for next upgrade

[legoktm]

Description

When testing a release, we usually just test the upgrade path to that release. In b46ee14 we discovered that the next upgrade would've been broken because of ordering in the postinst.

We should be able to test this with something like apt install --reinstall securedrop-app-code and then re-running the testinfra checks. This seems something that would be a small addition to the staging job but provide a nice coverage win so we're not running into issues during the next release.