networkd-tests.py: add helpers to common subprocess call patterns

networkd-tests.py: add helpers to common subprocess call patterns

For normal shell calls, python syntax is quite verbose. We don't need to punish
ourselves like that. In some places we would fork a shell to do argument splitting
for us. We know that our arguments can be safely split on whitespace, so let's do
that ourselves in all cases, without forking a shell.

We also expect command output to be valid text, so let's always set
universal_newlines=True.

This makes things shorter and easier to read. Development is also easier because
it's possible to paste many of the commands directly to/from a shell.