Actually validate network connectivity on NM MM setup

[asdil12]

Ticket: https://progress.opensuse.org/issues/155278
VR: https://openqa.opensuse.org/tests/3958923#step/setup_multimachine/45

[github-actions]

Great PR! Please pay attention to the following items before merging:

Files matching lib/**.pm:

• Consider adding or extending unit tests in t/

This is an automatically generated QA checklist based on modified files.

[ggardet]

Should we add a max number of retry?

[asdil12]

That should be covered by the assert_script_run timeout.

This will anyways either return on full or wait for full until a timeout occurs.