test: Fix networkd test for an already running service

test: Fix networkd test for an already running service

Mount tmpfses over the networkd and resolved config and state
directories, and stop the services beforehand. This ensures that the
test does not mess with an existing networkd/resolved setup. At least
for ethernet setups, this does not sever existing links, so is good
enough for the CI cases we are interested in (QEMU and LXC).

Relax the skip check to only skip the test when trying to run this on
real iron, but start running it in virtual machines now.

This allows us to run the test on Ubuntu 18.04 LTS in CI, which uses
both services by default.