Only capture the exec for `radiusd -v`, as facter etc. runs `uname -s` and whatever else. Mock Facter.value for things in os_facts, so we return the value from os_facts rather than the actual value from the system running the tests.

There is still a lot more to do here to get full coverage, and a lot of work to figure out testing across multiple OSes etc. however this is a solid start.