Add a test which runs nmcli against our stub NetworkManager
service and compares the output.
The output formats of nmcli are complicated and not easily understood.
For example how --mode tabular|multiline interacts with selecting
output-fields (--fields) and output modes ([default]|--terse|--pretty).
Also, there are things like `nmcli connection show --order $FIELD_SPEC`.
We need unit tests to ensure that we don't change the output
accidentally.
