piuparts update test fails when database is involved

Piuparts fails for bacula, see e.g. https://salsa.debian.org/bacula-team/bacula/-/jobs/5844264

The reason here is that at some point bacula-director-mysql is installed, which creates a database using db-config. After these tests have run, the package gets purged, but the database is not getting deleted. The next piuparts step is to test the update from the previous version of bacula-director-mysql to the version that's currently being tested. The installation part works for some reason, but the upgrade then involves a database upgrade which fails because it wants to create things that are already in the database because they were left over from the previous installation test.

It was confusing that piuparts works without error when packages for unstable are being tested. There is no error because the package update step (in this case stable->unstable) isn't done. I'm not sure if that is by design, but unexpected for sure.