CI: move deb package test to "test" stage
CI: move deb package test to "test" stage

This task depends on a target from the "build" stage, but nevertheless
it is still a "test" operation.

Later we should separate the deb package build from the upload to the
deb repository.

stadtgestalten/stadtgestalten,stadtgestalten/stadtgestalten,stadtgestalten/stadtgestalten