test: integration suite leaks `_default` networks (no teardown)

[Jaro-c]

After running the integration suite locally, the host is left with a large pile of orphaned _default networks:

$ podman network ls --filter label=podup.project | wc -l
~230   # t<id>-<name>_default from past test runs

The test teardown removes the project's containers but not the networks it created, so every run leaks one or more *_default networks. Over many runs these accumulate (a single podman network prune cleared ~230 here).

The integration harness should remove the networks it creates on teardown (or down should, and the tests should call it), so repeated local runs don't pollute the host. Not a runtime bug in podup itself, but it bites anyone running the suite on their machine.

[Jaro-c]

Low priority: this is test-only. podup down correctly removes the project's networks (verified against Podman 5.4.2), so users are unaffected. The leak is integration tests whose teardown doesn't remove the per-test project's network, accumulating orphaned t<id>-<name>_default networks on a contributor's machine. Worth a harness cleanup pass eventually; no user impact.