test: Add fake default route to work around podman 5 regression

[martinpitt]

podman 5 regressed user containers if there is no default route [1].
While that is being sorted out, add a fake interface with a default
route for our offline tests, to unbreak upstream podman PRs testing.

Same hack as in cockpit-project/cockpit-podman@cecb2cc6e8f2

[1] containers/podman#21896

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See this failure in cockpit-project/bots#6050

I trigger a fedora-40/other@bots#6050 run, but due to a bug in our new infra it will be reported as "fedora-40" (see pilot board).

[jelly]

Strange, there is an unrelated mypy issue here:

not ok 7 /static-code/test-mypy
# src/cockpit/transports.py:405: error: Argument 2 to "add_child_handler" of "AbstractChildWatcher" has incompatible type "Callable[[int, int], None]"; expected "Callable[[], object]"  [arg-type]
# src/cockpit/transports.py:405: error: Argument 2 to "add_child_handler" of "AbstractChildWatcher" has incompatible type "Callable[[int, int], None]"; expected "Callable[[], object]"  [arg-type]

[martinpitt]

@jelly yes, mypy "spontaneously" broke as the "tox" test does not pin versions. It's also on the pilot board. We need to move that into the tasks container and update it in a controlled way. It's very similar to new ruff versions, which I fixed last week (PR #20149)