autopkgtest: replace broken stubs with actual tests

Nis Martensenrequested to merge
nma/piuparts:autopkgtest into develop

Piuparts does not have --list (without further arguments) or --info options, hence running the autopkgtest currently fails. Replace the two broken stubs by two working tests:

  1. run piuparts --version
  2. create a minimal dummy binary package and run piuparts on it

Using a single-letter package name for the dummy package ensures that there is no package with the same name in the archive -- according to policy 5.6.1, all official package names must be at least two characters long.

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