Fix CI regression from 0bd26ee8 (Restructure tests)

0bd26ee8 added __init__.py files to the tests subdirectory. This had two
unwanted side-effects:

1. setuptools.find_packages() recognised the tests as a package, so they were
   included in the pip and Debian packages.
2. The Debian package build process with dh_python automatically runs tests
   with unittest. Unittest's test discovery (in contrast to pytest) only works
   if __init__.py files exist, so this step did not do anything in the past.
   Now, failing tests caused the whole CI pipeline to fail very early without
   the helpful information provided by later stages.

This change disables running any tests during the Debian package build. It also
explicitly sets the package list to "uffd".