Platform specific testing

[awvwgk]

I'm kinda run into this issue when creating unit tests for the fpm-install command. We make a lot of implicit assumptions on the OS which does not allow to test both variants on the same build machine. It would be helpful for unit testing (and caching) to give all filesystem operations an optional input to set the build machine OS. Testing this kind of stuff on the CI is not great.

Also this allows to cache the OS identifier minimizes the calls to get_os_type.

The alternative would be to always abstract all OS specific testing in a separate library, but this just shifts the problem away to another project.