unify packaging tests

Currently, there are two different implementations of packaging tests:

• distro/tests

  • depend on external repos (currently OBS)
  • use VMs, because we need to test systemd integration
  • are pretty much exactly what the user would use
  • primarily test default config

• tests/packaging

  • use docker containers
  • added in !892 (merged) and !947 (merged)
  • test if resolver starts up with given config
  • can't detect certain type of failures (crash shortly after startup, e.g. due to TA update)
  • test loading of optional modules

I think tests/packaging are redundant and they add additional maintenance burden (see my opinion in !947 (comment 154887)). However, they do test that all supported modules can be loaded in config. This functionality should be moved and unified with distro/tests as additional test, so we don't have to maintain two separate implementations of these tests.